The rudder is a board suspended below water at the stern of the ship used to steer the ship. The rudder is suspended from the sternpost at the stern above water to which is mounted a tiller, a board which runs inboard to control the rudder. The tiller is below decks.
In earlier times, several crewmembers were needed to operate the tiller and a quartermaster had to call down commands. Later, tackle was added with the tackle lines wrapped around the hub of the ship's wheel on the quarterdeck so that one or more helmsman could steer and watch heading, wind, and the sails.
The origin of the rudder was an oar, referred to as the steering board attached to the quarter of the ship, usually, on the starboard side, hence the term 'starboard'.