Basically it adds more co-op levels, challenge mode for singleplayer and co-op, and leaderboardsIn Peer Review, you and a friend will continue the story of loyal bots P-Body and Atlas as you puzzle your way through a mysterious new co-op test track and once again match wits with GLaDOS. The DLC also features a single player and co-op Challenge Mode, and leaderboards to compare Challenge Mode scores with friends and the Portal community.
Edit: It is free and portal 2 is 50% off today