As well as this camera-generated data, it is possible to edit metadata to include things like a caption, the copyright information and so on, which is saved with the image for sorting in the future.
When you take a photo with your camera, the image is written to the memory card for later editing on a computer. Alongside the image data, a whole bunch of other information is stored in the file. This metadata gives you essential information, like the date, time and camera settings relevant to each image. Metadata can be viewed in many image-editing programs, and Aperture is no exception, with the Metadata panel sitting prominently between the Library and Adjustments panes.