Back in Chapter 7, The Digital Darkroom, we introduced a few concepts that are crucial to becoming a selection expert.
Selections, channels, and masks are actually all the same thing in different forms. And you can convert one to another easily.
A channel is a saved selection and looks like a grayscale image where the black parts are fully deselected ("masked out"), the white parts are fully selected, and the gray parts indicate partially selected pixels.
A layer mask is a selection or channel applied to a layer so that the black areas of the mask fully hide the layer and the white areas of the mask are transparent (they show the layer's pixels). If an area in a layer mask (or channel) is 25-percent gray, then that area is 75-percent visible. Remember: "Black conceals, white reveals," and the lighter the gray, the more selected or visible the area.
Smooth transitions between selected (white) and unselected (black) areas are incredibly important for compositing images, painting, correcting areas within an imagein fact, just about everything you'd want to do in Photoshop.