Russ has pulled together a FANTASTIC comparison overview of Blip Messaging and Email,
At the heart of LLUP is the BLIP. The BLIP is a piece of content, declared in a standard way using XML, that specifies four main things:
* A reference
A pointer to the content that the BLIP describes.
* A lifetime
How long the BLIP will be relevant for.
Listed as keywords by a defined or undefined vocabulary. Can also include a signature or signatures of those who should receive the BLIP.
A means of signing the message by the message sender.
There’s more at the above linked reference.