The rudder is a primary flight control surface which controls rotation about the vertical axis of an aircraft. This movement is referred to as "yaw". The rudder is a movable surface that is mounted on the trailing edge of the vertical stabilizer or fin. Unlike a boat, the rudder is not used to steer the aircraft; rather, it is used to overcome adverse yaw induced by turning or, in the case of a multi-engine aircraft, by engine failure and also allows the aircraft to be intentionally slipped when required.
B727 Flight Control Surfaces. Source: Wikicommons. Origin: FAA(USA)