A rudder is a small flat board or blade that is attached to the stern (back) of a boat, ship or other watercraft. It is used as a means of steering and controlling the direction in which the vessel moves through the water. The rudder is connected to a series of ropes or cables which are controlled by the helmsman or pilot, allowing them to turn the rudder left or right, thus changing the course of the ship. In essence, the rudder acts as a "wheel" that helps guide and steer the vessel safely through its journey on water.