Image file formats provide a standardized method of organizing and storing image data. This article deals with digital image formats used to store photographic and other image information. Image files are made up of either pixel or vector (geometric) data (Wikipedia, retrieved 18:55, 30 July 2007 (MEST))
See also: Codec, screen capture tutorial, SVG, clipart, texture in this Wiki.
Unless someone decides otherwise, we probably will not expand this technical topic in Edutech wiki. Wikipedia articles for example, are fine enough. Below a few links:
There are several comparisons / overviews (some of which might be merged btw):