Layer Mask
A greyscale channel attached to a layer that controls where that layer is visible, with white revealing and black concealing.
In depth
Masks are the mechanism behind almost all local adjustment: rather than erasing part of a layer, the mask simply hides it, and painting white brings it back. Because grey values give partial visibility, a mask can blend an adjustment gradually — which is what stops local work from showing hard edges.
Examples in practice
Painting black on a contrast layer's mask over a subject's face keeps the increased contrast on the background only.