From SKYbrary Wiki
Ailerons are a primary flight control surface which control movement about the longitudinal axis of an aircraft. Differential ailerons function in the same manner as symmetrical ailerons except that the upward deflecting aileron is displaced a greater distance than is the downward deflecting aileron. As is the case with symmetrical ailerons, an upward deflection results in a reduction of camber and a corresponding reduction in lift on the affected wing whereas a downward aileron deflection results in an increase in both camber and lift. The increase in lift on the raised wing also results in an increase in drag which tends to rotate or yaw the aircraft around the vertical axis. In a differential aileron installation, the greater deflection angle of the aileron on the descending wing also induces drag which has the effect of balancing most (but not all) of the lift induced drag of the ascending wing. This greatly reduces adverse yaw.