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What Is...

What are Syndication Feeds

Welcome to the O'Reilly Network What Is site--the dedicated home to all of our "What Is XYZ" articles.

The "What Is XYZ" article is part of a new series we've started across all of our O'Reilly Network websites. Our goal is to answer this question for a number of technologies--the killer apps and hot new technologies of today, as well as the tried-and-true, foundational technologies--with high-level overviews.

Please check back here often, as we'll publish new articles in this vein regularly. And if you have a technology you'd like to see us cover in this manner, or that you'd like to write about, please let us know via email to


Articles sorted by: A-Z | Z-A | Publication Date | Subject

What Is the X Window System by Ellen Siever
Developed at MIT in 1984, the X Window System, now up to X11 release 6, or X11R6, has been the standard environment for Unix windowing systems. Ellen Siever provides some historical context for X's staying power, then discusses its major features: working with X and the X server and X clients; configuring X; and much more. Ellen is a coauthor of Linux in a Nutshell, 5th Edition. 08/25/2005

What Is Wireless Security by Swayam Prakasha
Wireless LANs have evolved into more affordable and logistically acceptable alternatives to wired LANs. But to take advantage of their benefits, your company's wireless network needs to be properly secured. This article covers the types of attacks wireless networks encounter, preventive measures to reduce the chance of attack, guidelines administrators can follow to protect their wireless LANs, and an excellent supply of online resources for setting up a secure wireless network. 03/30/2006

What Is a Wiki (and How to Use One for Your Projects) by Tom Stafford, Matt Webb
Wikis are becoming known as the tool of choice for large, multiple-participant projects because jumping in and revising the pages of a wiki is so easy for anyone to do. This article covers how to effectively use a wiki to keep notes and share ideas among a group of people, and how to organize that wiki to avoid lost thoughts, and encourage serendipity. Matt Webb and Tom Stafford co-authored this article using a wiki, as they did their book, Mind Hacks. 07/07/2006

What Are Web Parts? by Jesse Liberty
Today's web application is customizable in ways that could only have been dreamed of five years ago, partially because of Web Parts. Jesse Liberty shows how they work by guiding you through building a simple application in ASP.NET. Jesse is the author of Programming ASP.NET, Third Edition. 01/10/2006

What Is Web 2.0 by Tim O'Reilly
Defining just what Web 2.0 means (the term was first coined at a conference brainstorming session between O'Reilly and MediaLive International, which also spawned the Web 2.0 Conference), still engenders much disagreement. Some decry it as a meaningless marketing buzzword, while others have accepted it as the new conventional wisdom. Tim O'Reilly attempts to clarify just what we meant by Web 2.0, digging into what it means to view the Web as a platform and which applications fall squarely under its purview, and which do not. 09/30/2005

What Is Web 2.0 by Tim O'Reilly
Defining just what Web 2.0 means (the term was first coined at a conference brainstorming session between O'Reilly and MediaLive International, which also spawned the Web 2.0 Conference), still engenders much disagreement. Some decry it as a meaningless marketing buzzword, while others have accepted it as the new conventional wisdom. Tim O'Reilly attempts to clarify just what we meant by Web 2.0, digging into what it means to view the Web as a platform and which applications fall squarely under its purview, and which do not. 09/30/2005

What Is Vonage by James E. Gaskin
The modernization of telephone service--driven by Vonage and hundreds of competitors, including Ma Bell companies--is transforming voice communications as we know it. But what is Vonage? James Gaskin provides the answer, and looks at how Vonage is and isn't like traditional telephone companies. James is the author of Talk Is Cheap. 10/11/2005

What Is VoIP by Ted Wallingford
Voice over IP (VoIP) is the family of technologies that allow IP networks to be used for voice applications, such as telephony, voice instant messaging, and teleconferencing. Ted Wallingford looks at why the VoIP technology, the key ingredient in Vonage, Skype, Cisco CallManager, and a host of other technology products, is hotter than ever today. Ted is the author of Switching to VoIP. 09/02/2005

What Is Vlogging (and How to Get Started) by Josh Paul
Short for "video blogging," vlogging is another way to take advantage of the RSS enclosure tag. Josh Paul, author of "Digital Video Hacks," explains vlogging and shows you how to get your videos into the iTunes Music Store. 07/27/2005

What Is Visual Studio by James Avery
What can you really do with Visual Studio? James Avery discusses some of the various applications you can build using Visual Studio, some of its most compelling development features, and what you need to know to get started writing quality applications in Visual Studio. James is the author of Visual Studio Hacks. 08/22/2005

What Is Virtualization by Wei-Meng Lee
Virtualization lets you have multiple "virtual machines," each with its own operating system running in a sandbox, shielded from each other, all in one physical machine. But why would you want to do this? Wei-Meng Lee explains, and takes you on a tour of some of the most popular virtualization software available: Microsoft Virtual PC 2004, VMware Workstation 5.0, and Microsoft Virtual Server 2005. 12/06/2005

What Is Vim (It's Easier than You Think) by Matthew Russell
If you've had a less-than-agreeable experience with Vim in the past and are willing to give it another shot, please ponder these points: 1) there's generally a learning curve involved with any powerful tool, 2) Vim is much more than just your run-of-the-mill text editor, and 3) the effort required to become a proficient Vimmer isn't as much as you'd think: it's comparable to that of learning to touch type. Matthew Russell explains. 07/11/2006

What Is TurboGears (Hint: Python-Based Framework for Rapid Web Development) by Matthew Russell
TurboGears is a Python-based framework that enables you to quickly build database-driven, ready-to-extend web applications. In this article, Matthew Russell takes you inside this framework for a look at its internal mechanisms, then introduces you to its creator, Kevin Dangoor. 11/08/2005

What Is Struts by Chuck Cavaness
Chuck Cavaness takes you on a whirlwind tour of the Struts framework--an open source Java framework for building web apps--with overviews of many of it most important features, including Struts controller components, model layers, the Struts tag libraries, and presentation validation. Chuck is the author of Programming Jakarta Struts, 2nd Edition. 11/02/2005

What Is Spyware by Anton Chuvakin
As business use of the internet has grown up, so has business abuse. From this incubator emerged spyware -- the grownup cousin to viruses and worms, whose intent is not simply to have fun at someone else's expense but to make money at someone else's expense. Anton Chuvakin looks at what spyware is, how it works, and what you can do to keep your systems free of spyware infection. 11/22/2005

What Is Spring, Part 2 by Justin Gehtland,
In part one of this two-part excerpt from Spring: A Developer's Notebook, authors Bruce Tate and Justin Gehtland showed you how to automate a simple application and enable it for Spring. Today, the authors will cover how to use Spring to help you develop a simple, clean, web-based user interface. 10/12/2005

What Is Spring, Part 1 by Justin Gehtland,
In this first of a two-part series excerpted from Spring: A Developer's Notebook, authors Bruce Tate and Justin Gehtland help you understand how you can use Spring to produce clean, effective applications. In part 1, they take a simple application and show you how to automate it and enable it for Spring. 10/05/2005

What Is a Smartphone by Michael Juntao Yuan
A single device that can act as mobile phone and PDA is much better than having to carry multiple devices, hence the emergence of the smartphone. Michael Yuan discusses the evolution of the device. Then, for users, he details key features to look for when you're buying a smartphone. For developers, he describes the programming languages and APIs you can use to develop smartphone apps. Yuan is the author of Nokia Smartphone Hacks. 08/23/2005

What Is Skype by James E. Gaskin
Heard the Skype hype and want to learn more? Here's what you need to know. Skype lets you make free calls over the internet to anyone anywhere in the world who also has Skype. Your computer (or PocketPC) works as your telephone. In this article, James Gaskin descibes what Skype is, how it works, and where its limitations and strengths lie. James is the author of Talk Is Cheap. 08/04/2005

What Is Screencasting by Jon Udell
Whether it's for a tutorial, a how-to, or a software review, if you have a product you'd like to explain to other users, screencasting—the art of screen recording with audio narration—can be both easier and more effective than written descriptions accompanied by static screen shots. Jon Udell walks you through how to make, edit, and deliver compelling screencasts. 11/16/2005

What Is Ruby on Rails by Curt Hibbs
Ruby on Rails is an impressive web development framework that will soon reach version 1.0. While there's a lot of buzz, it can sometimes be difficult to discern the steak beneath the sizzle. Curt Hibbs walks through the features and pieces of Ruby on Rails to show how it fits together and where its big benefits come from. 10/13/2005

What Is Quartz (or Why Can't Windows Do That) by Matthew Russell
Quartz is the heart and soul of Mac OS X's graphics layer, which directly supports the defining features of the Aqua desktop experience. In this article, Matthew Russell gives you a short history lesson on Quartz, then shows you how it's implemented in Mac OS X. 10/11/2005

What Is Quartz by Chuck Cavaness
Java programmers: if you've ever needed an application to perform a task at a specific time, automatically, Chuck Cavaness suggests you check out the Quartz Scheduler. Cavaness looks at this open source job-scheduling framework, explains where to get it, how it works, and reviews its feature set. 09/28/2005

What Is Preview (and Why You Should Use It) by Giles Turnbull
Apple's Preview app is best known as a PDF viewer and a Macintosh alternative to Adobe Reader. This software, however, is also a capable image viewer, and you can even edit your pictures with its tools. Giles Turnbull shows you that there's more to Preview than first meets the eye. 07/19/2005

What Is Prefactoring by Ken Pugh
You've probably heard of Refactoring (the process of restructuring code without changing its external behavior), but what is Prefactoring? Ken Pugh provides the answer, then covers some of the guidelines to prefactoring in the areas of Extreme Abstraction, Extreme Separation, and Extreme Readability. Ken is the author of Prefactoring. 11/15/2005

What Is a Portlet, Part 2 by Sunil Patil
Portlets aim to be your next desktop, providing small pieces of web-based functionality that can be aggregated on a portal page. In this article, Sunil Patil delves deeper into the JSR-168 portlet spec by showing off edit mode, JSP integration, the portlet tag library and preferences API, and Pluto's admin console. 02/01/2006

What Is a Portlet by Sunil Patil
The Portlet API establishes a standard for building a web page out of smaller constituent parts, all managed by a portlet container to create a portal page. In this introduction, Sunil Patil shows how to create a basic "Hello World" portlet and deploy it in the Apache Pluto portal server. 09/14/2005

What Is a Portlet by Sunil Patil
The Portlet API establishes a standard for building a web page out of smaller constituent parts, all managed by a portlet container to create a portal page. In this introduction, Sunil Patil shows how to create a basic "Hello World" portlet and deploy it in the Apache Pluto portal server. 09/14/2005

What Is Podcasting by Phillip Torrone
So, you're ready to hop on the podcasting bandwagon, but you're not sure how to get started? This article by Phillip Torrone briefly describes what podcasting is and the software you'll need, then takes you right to the fun with a comprehensive step-by-step guide to podcast production. From recording to editing to publishing and syndicating your podcasts, Phillip covers everything you need to know to serve up your first podcasts. 07/20/2005

What Is a Pivot Table by Matthew MacDonald
Pivot tables are a hidden gem in Excel. While many otherwise experienced spreadsheet users avoid them because they seem too complicated at first glance, the real problem is that pivot tables are rarely explained properly. This article illuminates how your life will be better when you learn to use pivot tables, then walks you through how to build a basic pivot table. 08/12/2005

What Is a Pivot Table by Matthew MacDonald
Pivot tables are a hidden gem in Excel. While many otherwise experienced spreadsheet users avoid them because they seem too complicated at first glance, the real problem is that pivot tables are rarely explained properly. This article illuminates how your life will be better when you learn to use pivot tables, then walks you through how to build a basic pivot table. 08/12/2005

What Is Phishing (Or, How to Fight Phishing at the User-Interface Level) by Lorrie Faith Cranor, Simson Garfinkel
Phishing attacks use email messages and websites designed to look as if they come from a known and legitimate organization, in order to deceive users into disclosing personal, financial, or computer account information. This type of internet fraud will only grow more sophisticated in the days ahead, so our defenses against it must continue to improve. This excerpt from Security & Usability defines phishing, and offers techniques and advice on fighting phishing at the user-interface level. 10/25/2005

What Is OpenDocument by Sam Hiser
The OpenDocument Format (ODF), an open source file format standard for electronic office documents, is poised to change the world from an application-centric model of computing to a document-centric model. Sam Hiser looks at this new standard, how it implements XML for office documents, the technical and political wranglings in the standard, available tools, applications that offer ODF support, who's implementing ODF, and more. 07/27/2006

What Is Open Source by Dan Woods
Answering the question "What is open source?" used to be a lot simpler than it is today. Dan Woods provides some insight by first explaining how open source software is developed, then delving into how different groups define the term, and closing with a look at how open source institutions continue to advance the cause. Dan is coauthor of Open Source for the Enterprise. 09/15/2005

What Is On-Demand Computing by Stephen B. Morris
On-demand computing is a much-repeated term, but what does it mean, and what does it deliver? As Stephen Morris explains, autonomic computing, policy-driven workflows, and grid computing are all part of the answer. 11/30/2005

What Is NeoOffice/J (and Can It Replace MS Office) by Matthew Russell
NeoOffice/J is the long-awaited Mac-friendly version of OpenOffice. This open source project provides Mac users with most of the functionality of Microsoft Office, but for free. Is NeoOffice robust enough to serve as your only office suite in a Microsoft-dominated world? Matthew Russell explores. Plus, an in-depth interview with its lead developer, Patrick Luby. 08/09/2005

What Is a Linux Distribution by Edd Dumbill
The Linux kernel may be the star of the show, but like any star, it needs a supporting cast. In this case, the supporting cast is known as a Linux distribution--a useful set of system and application programs bundled with the OS. Edd Dumbill serves up overviews of the major Linux distributions as well as the specialist distros, and for who (or what) each distro is best suited. 10/27/2005

What Is the Linux Desktop by Jono Bacon
Much has been made of predictions about the "year of the Linux desktop," but what is the Linux desktop, why should we use it, and why should you care? Jono Bacon takes a look at development of the Linux desktop, from its roots to its apps to its future prospects. Jono is the author of Linux Desktop Hacks. 10/13/2005

What Is Linux by Ellen Siever
For a long time, Linux was seen as a geek's system--too complicated for ordinary folks. But Linux has matured, and with today's desktop environments and new user-friendly installations, Linux is finally coming into its own as a desktop system. Ellen Siever takes a fresh look at what Linux is (and is not), its features, distributions, and much more. Ellen is a coauthor of Linux in a Nutshell, 5th Edition. 10/06/2005

What Is Jetty by Q Ethan McCallum
Of course Tomcat is the first Java application server you think of, but is it the right tool for every job? The open source Jetty serves up JSPs and servlets in just a fraction of the memory needed by other app servers and is designed for easy embedding in other applications and non-traditional Java environments. Ethan McCallum takes a look at the big things in this small package. 06/14/2006

What Is Java by Chris Adamson
Everyone knows what Java is, right? Interpreted code, applets, proprietary, and slow. Wrong, wrong, wrong, and wrong. In its second decade, it's time to re-evaluate Java: the language and the virtual machine are going their own ways, its open source sub-community is vibrant and independent, and developers are taking the best ideas from other languages and frameworks and bringing them to Java. In this article, ONJava editor Chris Adamson tries to reset old assumptions about Java to fit modern realities. 03/08/2006

What Is an Iterator in C++, Part 2 by Ryan Stephens
In part one of this two-part series, Ryan Stephens described what an iterator is, both in terms of the iterator pattern and its implementation in C++. That explanation is sufficient only when you are using the standard containers, so in part two Ryan discusses the other kinds of iterators you should know about: reverse iterators, stream iterators, and custom iterators. Ryan is a coauthor of O'Reilly's C++ Cookbook. 11/21/2005

What Is an Iterator in C++, Part 1 by Ryan Stephens
Ryan Stephens provides an overview of the iterator pattern, then dives into what an iterator is in C++, first by looking at iterators in general, and then showing how to implement them in C++. Ryan is a coauthor of O'Reilly's C++ Cookbook. 10/18/2005

What is Java Content Repository by Sunil Patil
You might have heard of JSR-170, but what is a content repository, and what can you do with it? Well, do you want to manage documents with versioning, search, access control, and more? Content repositories offer these features, and JSR-170 codifies them into a single API. Sunil Patil shows how to use the reference implementation--Apache Jackrabbit--to create a blogging application. 10/04/2006

What Is Hibernate by James Elliott
Hibernate is a free open source Java package that makes it easy to work with relational databases. James Elliott describes the "enlightened laziness" that resulted in the development of Hibernate, how it works, and when it makes good sense to use it in your projects. James is the author of Hibernate: A Developer's Notebook. 09/21/2005

What Is Greasemonkey by Mark Pilgrim
Greasemonkey is a Firefox extension that lets you write scripts to alter the web pages you visit. Mark Pilgrim, author of Greasemonkey Hacks, shows how to install and use Greasemonkey to enhance your Firefox surfing experience. 09/01/2005

What Are Google AdWords by Sarah Milstein and Rael Dornfest
What are Google AdWords? What do all those sometimes confusing AdWord terms mean? What do you need to know before you sign up? If you're considering advertising on Google, this article has the information you need to know before you get started. 07/22/2005

What Is Google AdSense by Sarah Milstein and Rael Dornfest
Looking for ways to generate some cash for that website you've been developing? Google's AdSense may be your answer. This introduction to AdSense will help you decide if the program, which allows you to sell advertising space for other people's ads on your own site, is right for you. 07/26/2005

What Is the GNOME Desktop by Aaron Weber
Nope. It's not some elfish paperweight for the home office. GNOME is a desktop software environment designed to look familiar to anyone who has ever used a computer. Aaron Weber distills what the GNOME desktop is, what apps users will find as well as what platform development tools developers will find, and the resources to help you get started using it. Aaron is a coauthor of Linux in a Nutshell, 5th Edition. 08/18/2005

What Is Geronimo? by Kunal Jaggi
Geronimo is Apache's open source, Java EE-compatible application server, based on a flexible system of interchangeable components. This makes it ideal for a wide range of enterprise deployments. In this article, Kunal Jaggi shows you how to get started with Geronimo. 07/19/2006

What Are Generics by Jesse Liberty
Generics provide the ability to create type-safe collections in .NET. Jesse Liberty explains why they're important, and how to best make use of them. Jesse is the author of Programming ASP.NET, Third Edition. 11/28/2005

What Is Free Software by Karl Fogel
Today, free software is a large body of high-quality code on which much of the internet depends for critical functions. But free software is much more than a collection of programs. Karl Fogel examines free software under three different lights: as a political movement; as a programming methodology; and as a business model. Karl is the author of Producing Open Source Software. 09/29/2005

What Is Flickr (and Hot Tips for Using It) by Giles Turnbull
Flickr is an online photo management and sharing application. And it's also one of the most innovative photo services available today. In this article, Giles Turnbull shows you how Flickr works, then introduces you to some of the great tools you can use to interact with it. 08/02/2005

What Is FireWire (and How Best to Use It) by FJ de Kermadec
FireWire is a high-speed, data serial interface that has many practical uses. In this article, you'll learn the ins and outs of this powerful technology, plus some inside tips to take advantage of its unique features. 07/26/2005

What Is Firefox by Brian King
Brian King provides a brief look at Firefox's origins and evolution, and then dives into its support for web standards like CSS and XML, its debugging and extension capabilities, and some cool new features in the upcoming 1.5 release. If you're considering a switch to Firefox, this article may help make the decision for you. 09/26/2005

What Is Darwin (and How It Powers Mac OS X) by Matthew Russell
Darwin provides the underlying foundation for Mac OS X. In this article, Matthew Russell takes you to the core of Apple's OS and explains how it powers your Mac. 09/27/2005

What Is ClamXav (and do Mac users really need antivirus) by FJ de Kermadec
Do Mac users need virus protection? We believe yes. In this article F.J. introduces you to ClamXav, a free, open source antivirus application for Mac OS X. And as a bonus, we're including an interview with the developer who added the GUI to the excellent ClamAV engine to create ClamXav--Mark Allan. 08/19/2005

What Is C# by Jesse Liberty
Jesse Liberty reveals this little-understood secret: C# is really one of two "coatings" of MSIL, the Microsoft Intermediate Language (the second is Visual Basic 2005). Both C# and VB 2005 produce MSIL, and it is MSIL that runs on the .NET platform. Jesse provides an overview of the C# language and how it works within the .NET platform, and concludes with resources for coding in C#. Jesse is the author of Programming C#, 4th Edition. 10/03/2005

What Is Business Process Modeling by Michael Havey
Business Process Modeling (BPM) is a set of technologies and standards for the design, execution, administration, and monitoring of business processes. In this article, Mike Havey, author of Essential Business Process Modeling, briefly describes the state of BPM today and the BPM standards, then builds an ideal BPM architecture using the example of a retailer process. 07/20/2005

What Is Bluetooth by Michael Juntao Yuan
Bluetooth--its name is cool, but what can you really do with it? If you're curious, let Michael Yuan take you on a tour through the myriad use cases for Bluetooth-enabled devices, from car kits to social networking. He also provides overviews on the technology behind Bluetooth, and how to use it. If you're considering Bluetooth, either for app development or to create your own cable-free personal area network, this is a good place to start. 11/18/2005

What Is a BlackBerry by Dave Mabe
Wondered what a BlackBerry really does, and whether you should have one? Dave Mabe looks at six reasons why the BlackBerry is a true productivity tool, from its push email capabilities to its corporate customer features, and more. Dave is author of the upcoming BlackBerry Hacks. 09/15/2005

What Is Automator (and Can It Make Your Life Easier) by Matthew Russell
Automator provides an intuitive drag-and-drop workbench for quickly streamlining repetitive tasks. Here's a look at creating workflows and working around Automator's inherent limitations. 09/06/2005

What Is Asterisk by Brian McConnell
Asterisk is an open source PBX (private branch exchange) that provides all the functionality of high-end business telephone systems, and much more. Brian McConnell explains how Asterisk works and where to get it, and provides an overview of its feature set and its platform capabilities. 09/30/2005

What Is ASP.NET by Wei-Meng Lee
Part of the .NET Framework, ASP.NET allows developers to build dynamic web apps and web services using compiled languages like VB.NET and C#. Wei-Meng Lee provides a look under the ASP.NET hood, describing how it works, its improved support in areas like state management and tracing and debugging, and important new features in version 2.0. Wei-Meng is the author of ASP.NET: A Developer's Notebook. 09/19/2005

What Is Activity Monitor (or How to Take Your Mac's Pulse) by Giles Turnbull
This little utility is easy to overlook, but it comes pre-installed with every new Mac and can be a big help when you're running a lot of applications. In this article, Giles Turnbull takes you on a guided tour of Activity Monitor's features, and suggests circumstances in which you might want to make use of them. 10/04/2005

What Is .NET by James Avery
.NET is probably one of the more muddled and mismanaged brands in the history of Microsoft. Elucidator James Avery clarifies things by describing the two chambers at the heart of .NET: the Common Language Runtime (CLR) and its essential components, and the Base Class Library (BCL) and its major features. 09/06/2005