G-Sides is, as mentioned, an album of b-sides (or in this age of CD's, extra tracks) from the first Gorillaz album.
Whats particularly good about the album is getting hold of tracks that you would have missed other wise, stuff like the Scientist/Dub influenced "Dracula" is a great addition, with its heavy bass sound, and the mellow hip-hopp of "The Sounder".
This is a must have album for the Gorillaz completist (if there is such a thing) but if you have enjoyed even just one of their tracks, its worth picking this up to sample some of their other flavours.