Aperture’s Rotate tools, the ones that sit in the Toolbar at the top of the Aperture window, provide a simple facility for rotating images right or left. Select either the clockwise or counter-clockwise rotate tool and then click on any image and it will rotate 90°. If you hold down the Option key while clicking, you’ll get the opposite rotation from either tool.
However, if you select multiple images and click on one of them with the Rotate tool, only that image will rotate (regardless of the state of the Toggle Primary Select button). This might seem unintuitive, but don’t worry, there’s still a way to rotate entire groups of images. Just select the images as normal, and then use the ] key to rotate clockwise, and the [ to rotate counter-clockwise. Alternately, you can use the Rotate commands found under the Images menu.
Also, don’t forget that you can rotate single or multiple images upon import, by using the Rotate commands found in the Aperture Import dialog.