MusicBrainz is a user-maintained community music metadatabase. Music metadata is information such as the ArtistName, the AlbumTitle, and the list of tracks that appear on an album. MusicBrainz collects this information about music and makes it available to the public so that music players can retrieve information about the music that is playing. For instance, most audio CDs do not contain the name of the artist, album, or a listing of the tracks. A music player can use the digital characteristics of an audio CD to look up the correct metadata and show it to the user during playback.
(via Creative Commons Weblog)