Hard to describe but really is fun!
First and for most the games single player mode. You are Sam Fisher a secret agent and now turned to a double agent. this means you have to make choices to kill or not to kill that is the question. If you kill your alliance for the JBA will go up but your alliance will go down with the NSA and visa versa. Now some of the styles of taking your enemies to their knees (or their graves) varies quite a bit. You can do the splits on a wall and drop down on dumb enemies that walk under you. Or you could just shoot them from there. Also you can sneak up behind them and choke them out or snap the neck. You can be on a pipe when they walk under you can knock them out or..... well you get the point. So many was so little time to explain so go play it to find them all out but remeber every choice makes a difference so choose wisely ok. Then there is the multiplayer mode which is a great thing in its own right.
Mulitplayer has alot of different things you can an must do like climb on each others shoulders to cut camera's, Hurl one or the other over a gap or lasers, and much more in this the perpose of it is to believe it or not but help out sam fisher in his main storyline (single player mode). Its an important task and if you and your alli arent on the same page prepare for a lot of dying because that will be inevitable you got to litarally work as one cohesive unit. well i dont know how to explain any more so go play and figure it out trust me you will have fun while you do so i promise.