The aileron is a movable control surface mounted on the trailing edge (rear part) of the wing, and controls the roll axis
of the airplane. Normally there is one on each wing, and they are interconnected. When the pilot moves the stick to the left, or rotates the control yoke to the left (counterclockwise), the aileron on the left wing (left aileron) moves up, and the right aileron moves down. When the stick/yoke is moved to the right, the right aileron goes up and the left one goes down.
In either case, the upgoing aileron reduces the lift
of that portion of the wing, making it want to go down, and the downgoing aileron increases the lift on its portion of the wing, making it want to rise. This is how you roll an airplane, or make it bank.
A side note
: Biplanes may have either two or four ailerons. With two ailerons, there is one on each side on the lower wing (occasionally it may be on the top wing, instead). With four ailerons, there is an aileron on each side on both top and bottom wings, increasing the roll response of the aircraft.
In addition, many jets have an outboard aileron on each wing, and another inboard one. The outboard one often isn't used at high speed, but comes into play at slower speeds (for better control). Many jets (and some other aircraft, such as the Mooney MU2) will have ailerons for roll control. This may be in addition to ailerons or instead of them.
the Wikipedia entry aileron