Spoilers - Typically used in landing gliders or sailplanes, these are on the
top of the wing. When these are raised, they reduce, or spoil the lift of the
wing in that area. They can help slow a plane down and raise the stall speed
of the plane causing it to descend from lack of wing lift. These can also be
helpful in getting sailplanes out of strong thermals.

You can see the spoilers opened on the top of the wings of my Spirit.