This game has a deep psychological effect, that in all fairness is very scary. This games tests your reflexes and your logical thinking, there are clues that you have to look or listen for, such as footsteps or gunshots or a letter on the wall or desk.
You can get a range of weapons from a knife to a rocket launcher, each have their own use and destructive properties. One thing that will help you to save ammo, which will save your life later in the game, is that if you can run past it do not bother shooting it, but do not try this hunters.