Like many terms use in aviation "slots" and "slats" can mean more than one thing according to applicability and historical pedigree! I'll stick to the common understanding of the terms as I've come to know them. While I have simplified my explanations somewhat, it's my hope that I've not strayed so far away from the objectively academic so as to promote any misunderstanding.
Most commonly, a slot is a gap through which higher pressure air can flow from the lower airfoil surface to the upper surface. As the air passes through the slot it increases in velocity and is added to the airflow over the upper airfoil surface. This adds energy to the boundary layer airflow over the upper surface, thus increasing the critical angle of attack in the area behind the slot. By delaying the onset of airflow separation to a higher AOA, slots increase the maximum coefficient of lift attainable across a particular airfoil section.
Slots may be found placed slightly aft of a wing leading edge (fixed slot), between a leading edge slat and the wing leading edge or between trailing edge flap segments on a multi-segment trailing edge flap system such as Fowler flaps.
The fixed slot is typically placed along the outboard section of a wing leading edge. Placed as such, the critical AOA is higher in the outboard portion of the wing than the inboard portion, thus promoting inboard to outboard stall progression and maintaining aileron effectiveness to a higher AOA. This is especially useful in reducing the roll moment produced by asymmetrical stalls. Wing washout and and stall strips are two other methods available to accomplish similar results on a constant chord wing, though perhaps somewhat less effectively.
A slat may be fixed or moveable. A slat extends forward and downward into the airstream. If fixed, it is sometimes referred to as a leading edge "cuff". If moveable, slats may be actuated by a powered aircraft system such as hydraulic or electrical. Slat rails are placed at intervals along the wing span and oriented longitudinally to guide the slat along it's travel path. A slat is to a droop leading edge as a Fowler flap is to a plain flap. DLEs and plain flaps simply hinge down, increasing wing camber, while Fowler flaps and slats extend downward AND transit along the chord line. Both the Fowler flap and the slat increase the wing surface area in addition to increasing the airfoil camber as they extend.
Alternatively, slats may be actuated aerodynamically when high AOA causes an air pressure difference which acts to pull the unpowered slat out on it's rails to the extended position. Dynamic pressure pushes them back to the stowed position when the AOA is reduced enough. North American aircraft such as the F-86 Sabre and T-39/Sabreliner (NA-265) were equipped with such a slat system, sometimes erroneously referred to as "gravity slats". It's a matter of some interest that asymmetric deployment/stowing was not considered to be a significant matter of concern among pilots, though a slight rolling motion was not uncommon during deployment.
I hope that's helpful,
westhawk
Last edited by westhawk; 14th Jan 2013 at 08:06.