| A speed brake is a device used for lowering the velocity of an aircraft in preparation for landing, or sometimes for drastic speed changes (or to prevent same) during a combat or aerobatics maneuver. They are sometimes known as 'dive brakes' if their purpose is to allow the airplane to dive at steep angles without gaining too much speed (the Junkers Ju 87 Stuka was an early adopter). They perform this function by increasing the drag on the aircraft and hence causing it to shed energy. One method is to use dedicated trailing edge wing flaps which extend backwards and downwards sharply, at enough angle such that they cause much more drag than excess lift (which is what regular flaps would do). Usually, these are kept near the wing root so as to minimize their interference with control surfaces. The aforementioned Stuka had brake panels which would swing down from the wing into the airflow, separate from the control surfaces. The actuation of these dive brakes would also activate a rudimentary dive autopilot, placing the airplane into the proper dive to allow the pilot to concentrate on aiming his bomb release.
Another method is to use some of the same control surfaces as the flap systems, but to have a 'speed brake mode' which pushes them further or to a sharper angle than they would be for 'flaps' settings. Typically, speed brakes of this type are only used on the ground, as changing the aircraft's dynamics during landings could lead to severe trouble. If you are in an airliner, and immediately as the wheels touch you hear the whine of hydraulics in the wings (not the engines) it may be speed brakes pushing out. Note that these should not be confused with spoilers, which are typically extended upwards from the wing top, or forwards and down from the wing front; these devices, rather than slowing the aircraft, are specifically intended to decrease its lift by 'spoiling' the shape of the wing.
Of course, speed brakes don't have to be in the trailing edges of the wings at all. Some aircraft either don't have the trailing edge space to spare, needing it all for control surfaces (and not being able to spare it for dedicated brakes, and needing brakes in flight). Since speed brakes need do nothing more than increase drag, simply shoving a piece of the skin out into the slipstream will work, so long as it doesn't destabilize the aircraft. The F-15 Eagle and some other fighters, for example, tend to have a single large airbrake surface which is pushed up at an angle from their middle dorsal surface by a piston, looking sort of like a ramp. The higher the rear of the ramp is pushed out, the more the brake takes effect. Some alternate versions of this approach push diverters out on either side of the fuselage.
Rather than having diverters blossom from the fuselage, some designs have them sprout from other places. The Space Shuttle Orbiters have speed brakes which appear from either side of the rudder, making it appear as if the rudder has split at the back edge. This is possible due to the large size of the rudder and vertical stabilizer aboard the Orbiter, as well as due to the low relative cross-section increase required to slow the Orbiter usefully as it performs its energy management before and during landing. For some aircraft, especially those which only require speed brakes for landing, extending the landing gear alone can safely and usefully retard their flight enough to serve as a speed brake, and they are designed to accommodate that purpose.
One thing speed brakes do have in common is that they cannot usually be deployed above a fairly low speed without endangering the airplane, and will typically have safety systems to prevent them doing so. Extending-flap versions will usually refuse to function unless there is weight on the landing gear (i.e. the airplane is on the ground). | 
