Aileron in RC Airplanes: A hinged control surface on the trailing edge (rear side) of an airplane wing near the tip, the aileron moves up and down and controls the direction of a rolling turn. An airplane has a pair of ailerons, controlled by servos, that move opposite of each other unless they are in the neutral (flat with the wing) position. With the right aileron up and the left aileron down the airplane will roll to the right. Move the right aileron down, the left goes up and the airplane starts rolling to the left.
Aileron on RC Helicopters: Although RC helicopters don't have ailerons, the term is also used in RC helicopter flight to describe the controls for left and right banking turns. The aileron control is another name for the cyclic roll -- left or right banking or rolling to the left or right.
Aileron Control on an RC Transmitter: On a typical 3-channel or 4-channel RC transmitter, movement of the ailerons (if present on the RC) is controlled by the left / right movement of the right stick. However, some controllers may use a different configuration.