Flaps provide lift and drag.

Wikipedia.org describes this better than I can:

Flaps are hinged surfaces on the trailing or leading edge of the wings of a fixed-wing aircraft which, when deployed, change the coefficient of lift and drag of a wing.

Lift is increased by:
* increasing the camber of the wing.
* increasing the size of the effective lifting surface by increasing the wetted area.

Drag is increased by:
* increasing the wetted area of the wing, resulting in more skin friction drag.
* increasing the induced drag of the wing.

Flaps are usually fully extended while landing to allow the aircraft to fly more slowly (by increasing the lift generated by the wings at slow speeds) and to steepen the approach to the landing site. Depending on the aircraft type, configuration and method of takeoff (e.g., short field, soft field, normal, etc.), flaps are often partially extended for take-off to give the aircraft more lift when trying to leave the ground.