Doors are environmental objects usually found near the entrances of the structure. Doors are rather fragile and in addition can be destroyed by removing ground underneath them.
For enemies, closed door acts as a solid terrain which they can't see though.
Doors are not opened by grenade impacts (but are still vulnerable to the explosions).
Doors are opened automatically when player crosses the doorway. Weak enemies like Basic Mooks are killed when they they are in the way of a door opening.