A doorjamb or "door jamb" is part of a door's frame. The door jambs are the two long, upright pieces on either side of the door frame.
Jambs are an important part of a door's frame, as they support the weight of the door by way of the hinges. They also hold the openings that the bolts and latches extend into when they are locked. Each of the jambs has its own name -- the "hinge jamb" on the side that holds the hinges, and the "strike jamb" on the side where the locks are located. In British English, they are sometimes called "doorposts."