I've heard of a lot of mods, but I'm still not sure what they are (they should have something on mods in the manual). Are mods modifications in the core of the game? or are they just patches you install on top? And how much can they change of the game? Can they totally change the game (multiple guns can be caried, waking on walls for humans, flying without jetpacks, faster shooting, infinite ammo, etc.) or do they change only superficial thing?