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I always thought that the definition of slat was:
"Lift modifying and adjustable wing element on the leading edge of main airfoil."
and the definition of flap was:
"Lift modifying and adustable wing element on the trailing edge of main airfoil."
However, there are also leading edge flaps, also known as Krueger flaps, and they are located on the leading edge.
What gives these elements the name of "flap"? What is the main difference to slats?
Slats are leading edge devices on aircraft that enable higher angles of attack. Flaps are devices on the leading (Krueger) and trailing edge which increase camber and the depth of the wing. Slats create a "slat" between the slat body and the wing leading edge so the air can pass from the bottom to the surface to hold off a stall to a higher AoA, flaps merely extend it fore and aft and direct the flow downwards for more lift. Both are mainly used in the low-speed situations of take-off and landing.
First of all, slats produce a slot (not slat) somewhat aft of the leading edge in order to allow high-speed air from below the wing to pass through and maintain laminar flow to a somewhat higher angle of attack. They are a low-speed device.
There are also mid-wing slats which both produce drag and decrease lift and are used to slow down and get down, and, used on one wing at a time, cause roll. They may be used at high speeds.
Both trailing edge flaps and LEDs (leading edge devices, be they leading edge Kreuger flaps or slats) increase camber in order to allow the wing to produce the same amount of lift at a lower airspeed (takeoff and landing). They do not increase lift; if they did the aircraft would go up (and slow down rapidly) which is not the point.
Not all leading edge flaps are Kreuger flaps, which rotate out from the lower forward wing surface. There are simple hinged leading edge flaps, as well. The example with which I am most familiar is the Chance-Vought F-8 Crusader. The entire leading edge droops simultaneous with trailing edge flap extension -- the inner wing section 25 degrees, the outer section 27 degrees on all US models except the F-8J. Lower stall speed and carrier approach speed are the reward. ref: F-8H NATOPS Manual.
