A winch is a rotary device used to control a line. A winch consists of a cylindrical drum that is ratcheted so that it can only turn in one direction (usually clockwise), internal gears, and a handle that can usually be removed.
When a line is wrapped around the drum, friction helps prevent the line from paying out. The gears are configured such that turning the handle allows the user to apply greater tension on the line than he or she could do so by hand. Tension must be kept on the tail end of the line to prevent slipping; this is called tailing.
Many winches have two speeds, such that turning the handle in opposite directions turns the drum in the same direction, but with different gear ratios.
Self-tailing winches have a slot at the top in which the line can be secured, which eliminates the need for tailing.