Spoilers are control surfaces which spoil the lift and increase the drag of a wing by disturbing the airflow around it.
The spoiler causes the airflow over the wing to separate from the portion of the wing behind the spoiler, greatly reducing the lift generated by that wing section. Spoilers fall into two categories: those that can be deployed at controlled angles during flight to increase descent rate or control roll, and those only intended to be used on landing to quickly reduce lift and increase drag. Spoilers that can be used in flight are known, naturally, as flight spoilers (spoilers used to provide roll control are also known as spoilerons or roll spoilers), while those used only on the ground are known, surprisingly enough, as ground spoilers. (Most flight spoilers also function as ground spoilers to some extent, as their maximum deflection is limited in flight, with the spoilers only popping up to their fully-extended position upon touchdown.)
Spoilers differ from airbrakes in that airbrakes are designed to increase drag without affecting lift, while spoilers reduce lift as well as increasing drag.