There's four difficulty options (Novice, Stalker, Veteran, and Master I think?).
The differences between them from what I've hears the percentage of the time that bullets will do damage, for the player and the NPCs. So from my understanding on lower difficulties some of the bullets that you and your enemies shoot are kind of ghost bullets that don't do damage, and on Master all of them can hurt things.
So on the lower difficulties the game might let you run around a bit more, but on the higher difficulties it might slow the pace down and make you use things for cover more.