Create a room with a maximum number of players, public or private etc. I do not want talk about unique maps, weapons, or multiplayer mode, many of these stuff are also in other games.
Use WASD to move. Spacebar to jump. T to chat. Mouse to shoot. Shift to run. P to pause the game. Enter to respawn. G to throw a grenade. E to pick up a gun. F to use a knife. R to reload your weapon. C to crouch. 2 to change your weapon. Arrow keys for kill streaks. Tab key for the score.