- SELECTIONS, LAYERS, and MASKS
ABCs OF MASKS
The three major "concepts" in Photoshop can be treated as a mask.
SELECTIONS
A SELECTION is "transparent" mask to the underlying pixels of the active layer.
LAYERS
A LAYER is a "stacked" mask to the underlying pixels of the layers below.
MASKS
A MASK is an "actual" mask to the underlying pixels of the layers below.