A rudder is a flat piece of material, usually made of wood or metal, that is attached to the back of a boat or ship. Its primary function is to steer the vessel by directing the flow of water. When the rudder is turned, it creates a force that changes the direction of the boat, allowing it to navigate through water. Rudders can vary in size and shape depending on the type of vessel, from small kayaks to large cargo ships. They are often controlled by a steering wheel or tiller, which allows the operator to maneuver the boat effectively.