For questions about those devices that swing forwards and downwards from the leading edge of an aircraft's wings, allowing them to produce more lift without stalling. Questions about high-lift devices in general should use "high-lift-devices" instead.
A slat is a type of high-lift-device attached to the leading edge of an aircraft's wing. It slides forwards and downwards as it deploys, opening up a slot in the wing behind the leading edge; this alters the airflow over the upper surface of the wing, and allows the wing to fly at a higher angle-of-attack (and, thus, produce more lift) before it begins to stall. Most types of slats also increase the curvature of the wing (known as its "camber"), which increases the wing's lift directly by a small amount.
There are three main types of slats, distinguished by how they are operated:
- Powered slats are extended and retracted by the pilots, either with cables or by means of the aircraft's hydraulic-system.
- Automatic slats are spring-loaded; at cruising speed, ram air pressure holds the slats in the stowed position, but, when the aircraft slows down to land, the pressure on the slats decreases and the slats pop open.
- Fixed slats, also called slots, are permanently fixed in the deployed position; this is the simplest system, but creates a large drag penalty at high speeds, and is, thus, generally only used on low-speed aircraft.
Attempting a takeoff with an aircraft's slats retracted is extremely dangerous, and has resulted in a number of fatal accidents. With the slats retracted, the aircraft can easily stall as the pilot rotates for liftoff; even if it doesn't stall, the aircraft will have to accelerate to much higher speeds before it can lift off, potentially leaving little or no room for a rejected-takeoff should something bad happen.
For more information, see the Wikipedia page on slats.