From SKYbrary Wiki
Ailerons are a primary flight control surface which control movement about the longitudinal axis of an aircraft. This movement is referred to as "roll". The ailerons are attached to the outboard trailing edge of each wing and, when a manual or autopilot control input is made, move in opposite directions from one another. In some large aircraft, two ailerons are mounted on each wing. In this configuration, both ailerons on each wing are active during slow speed flight. However, at higher speed, the outboard aileron is locked and only the inboard or high speed aileron is functional.
Moving the flight deck control wheel or control stick to the right results in the aileron mounted on the right wing to deflect upward while, at the same time, the aileron on the left wing deflects downward. The upward deflection of the right aileron reduces the camber of the wing resulting in decreased lift on the right wing. Conversely, the downward deflection of the left aileron results in an increase in camber and a corresponding increase in lift on the left wing. The differential lift between the wings results in the aircraft rolling to the right. On some aircraft, ailerons are augmented by roll spoilers mounted on the upper surface of the wing.
In the functional example above, the increase in camber of the left wing results in an increase in lift but this, in turn, also causes an increase in drag. This added drag causes the wing to slow down slightly resulting in rotation, referred to as yaw,around the vertical axis. To overcome this yaw and thereby maintain coordinated flight, rudder input is required while entering and exiting a turn. To minimise the amount of adverse yaw produced during a turn, engineers have developed various aerodynamic and mechanical solutions including differential ailerons and coupled ailerons and rudder.