Portal 2 is the sequel to the critically acclaimed Orange Box hit Portal. Portal 2 brings all new game mechanics as well as co-op play. Portal 2 expands upon it's predecessor taking the same concept but turning it into a full length, full price, stand-alone game. This time it is not accompanied with the Orange Box, yet still has been developed and published by Valve.
Main game play consists of 'shooting' portals onto walls and structures so you can travel from one place to another via these portals. You move throughout 'test chambers' in which you complete tasks that ultimately lead to the next chamber. As the game progresses chambers get harder and you need to find more innovative and precarious ways of reaching the end. Use of momentum in Portal 2 is crucial.
The story is in close relation with the original, you still play as the same character as the original but set many years after; you've been kept sedated in stasis. You are still in the same test facility but the environment is very dilapidated.
The game may have kept GLaDOS but they have added more characters including Wheatley a spherical robot that helps you throughout the game (no spoilers). He is voiced by Ricky Gervais' co-writer Steven Merchant and provides brilliant voice acting to the game. In true Valve style there are no cut scenes as such so voice acting and writing play a huge part, and has hit the nail on the head.
As mentioned before the game does feature a cooperative game mode for up to two player. You play as Atlas and P-Body, two robots that are also venturing throughout 'chambers'. They use entirely new maps that are mostly if not all dependent upon cooperative play; a headset is a necessity. In a lot of situations all four portals will need to be placed thought out the chamber to continue on with the game.