Wikimedia
Wikipedia: Raster Graphics (Bitmap)
Encyclopedia entry that defines what a Raster graphic (Bitmap) is and explains how it works. Includes a nice illustration and an example of how a bitmap picture is constructed. Links to related information are available.
Other
University of Edinburgh: Glossary of Introductory Computer Graphics Terms
This website, from the University of Edinburgh, provides a lengthy glossary of computer graphics terms written by Olin Lathrop, author of "The Way Computer Graphics Work". This site provides simple definitions of basic terms; no graphics...