Krugers (should be an umlout over the U but can't do it on my keyboard in pprune) are the simplest form of LE device. They are simply a panel which folds out forward. The variable camber devicesa are flexbible composite panels which lie flat whe stowed, but flex into a curved shape to cmplement the natural shape odf the aerofoil when deployed.
Slats aren't fitted to 747s. Technically, a slat is part of the aerofoil which moved forward of the wing to extend the leading edge forward and downward. Airbusses use slats, they tend to be more difficult and expensive to construct. They can also have slots which allow some of the higher pressure air on the undersurface to leak to the upper surface which re-energises the upper surface boundary layer thus delaying the onset of separation and delaying the stall.