Wikimedia
Wikipedia: Jpeg
Encyclopedia entry that defines a JPEG (Joint Photographic Experts Group) graphics file and explains how and why it was created. Links to related information are available as well.
Other
Elated Communications: Understanding Image Formats
This tutorial looks at the three common image formats used on the web: GIF, JPEG and PNG. You'll learn how these image formats work, and where to use each type of image in your web pages.