A damper is a movable plate or valve used to control air flow. In a fireplace or wood stove, a damper is used to control the amount and direction of air going to the fire. It must be fully open before attempting to light the fire. When the fireplace is not in use the damper should be closed to prevent air from being drawn up through the flue.
The commonest kind of damper is a throat damper, which is located in the throat of the fireplace, just above the firebox. It is operated by means of either a handle inside the firebox or a knob above the fireplace opening which is connected to a rotating metal shaft attached to the damper.
A top-sealing damper is a damper located at the top of the chimney and is operated by means of a cable extending down the flue to a handle in the firebox. Often, if the chimney has been without a cap, the damper will be quite rusted, in which case installing a top-sealing damper may be a good idea.