Arma 2 is 1st person FPS based on military/war type combat. It's a more technical game than Call of Duty for example. You don't just run around like an idiot spraying bullets everywhere.
However, last I tried to play the game (several months ago) it had severe performance problems even on high end machines. So you'll want to check out the Arma 2 forums before you try playing, just to see what's up these days.