RC Airplane Rudder: The rudder is a hinged control surface on the vertical stabilizer or fin at the tail of an airplane. Moving the rudder affects the left and right movement of the airplane.
The airplane turns in the same direction that the rudder is turned. Move the rudder to the left, the plane turns to the left. Move the rudder to the right, the plane turns to the right.
On a typical 3-channel RC airplane transmitter, the left / right movement of the right stick controls the airplane rudder. On a typical 4-channel RC airplane transmitter, rudder control is on the left stick. Other configurations are possible.
RC Helicopter Tail Rotor: An RC helicopter doesn't have a rudder but the tail rotor can act in a similar fashion as the rudder on an RC airplane and the tail controls are often referred to as rudder controls. Changing the pitch of the tail rotor on the RC helicopter causes the heli to turn to the left or right, much like moving the airplane rudder to the left or right initiates a turn.
On a typical RC helicopter transmitter, the left / right movement of the left stick controls the tail or rudder. Other configurations are possible.
RC Boat Rudder: On radio controlled boats the rudder is a vertically hinged plate at the rear or stern of the boat that can be turned to steer the boat left and right. Move the rudder to the left to turn the boat left. Move the rudder to the right to turn the boat to the right.