A site that provides some good pages discussing various fundamental aspects of Data Compression. Although the site appears to be a university project in Croatia, all of the pages are writen in English.
This link points to an archived site, as the original has disappeared. Links on the archived page may or may not work properly.

http://web.archive.org/web/20020604012435/http://www.rasip.fer.hr/research/compress/index.
html