Doors are usually a barrier to another room.  Depending on one's
perspective, they can be extremely effective protection against
whatever is in that room, or an extremely annoying deterrent in
preventing you access to the goodies on the other side.  Doors
can be opened, closed, locked, broken, or jammed.  Some doors
can be picked or bashed.  Doors come in a variety of types, 
including cloth, wood, hard wood, banded, plate, thin metal, thin
stone, thick metal, thick stone, gemstone, and magical force.  
Some doors have traps, so be careful.