A Master Track is the original recording of a song by an artist. All songs on CD, record, cassette, or any other form of media are based on this recording.
This is necessary for the Guitar Hero series because on a master track all instruments are separate tracks, which is required so the guitar sounds can be separate from the rest of the music.
In Guitar Hero II, a master track is shown by Performed by such as Stop as Performed by Jane's Addiction. In Guitar Hero III, a master track is indicated as "by". Since Guitar Hero World Tour, all songs have been master tracks.
Generally, all old songs in Guitar Hero 3 are covers, with the expection of Paint It, Black.