ORIGINAL: rb4123
As cafeen man said, spoilers break up the smooth airflow over a wing, therefor decreasing lift on that wing and causing it to drop. By dropping the inside wing, and not trying to raise the outside wing(as in inside and outside of the turn), you also avoid adverse yaw caused by the increase in lift of the outer wing.
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Three types of spoilers. They do the same thing, but are used for different reasons. In fact, many times one board of a spoiler can be used for all 3 of the functions, but that is not always the case.
Ground spoilers: Deploy after touchdown to kill all lift on an airplane after it has landed. Usually on larger airliner types or smaller jets and some turboprops.
Roll spoilers: They aid in rolling the airplane. If turn the yoke/stick to the right, the right roll spoiler will come up. That decreases lift on that wing and causes the a/c to bank to the right. Roll spoilers limit the amount of adverse yaw that is associated with just using ailerons.
Flight spoilers: They are used to help decend the a/c without picking up speed. They aren't speedbrakes, exactly, but more like lift-killers. They can make an airplane come down like a piano.
Like I said, quite a few airplanes will have all three types of functions on the same spoiler panel, or just one or two panels will be used for roll and flight spoilers but the other 5-6 will come up for ground spoilers during landing.