CDDB is a database to look up CD (compact disc) information using the internet. This is done by a client which calulates a (nearly) unique disc ID and then queries the database. As a result, the client displays the artist, CD-title, tracklist and some additional infos.
An alternative project that aims to enhance CDDB beyond a mere database of CDs is available at http://www.musicbrainz.org. This site also contains more information on CDDB and some database statistics of CDDB, FreeDB, and MusicBrainz.