You can think of a mask as a kind of covering that goes over an image. This description may sound a little bit like a layer, but the reality is that masks are quite a bit different.

When you apply the colour white to part of a mask, you will remove that part of the mask, and the underlying layer or image will be exposed in these areas. If you apply black to a mask, the underlying image will not be exposed wherever the colour black is applied. You can apply this black or white colour with a paint brush, a gradient tool, or even a pencil tool. This means that you can literally paint on and off the parts of your image that you want exposed.

Notice how the paintbrush strokes using the white foreground colour have removed the mask.

With a mask, only the exposed area of the image can be edited. You can use paintbrushes to apply the colour white, and in so doing, fine tune what parts of your image will be exposed to edits or effects. If you want to cover up part of an image so it is not exposed to editing, you can paint what you want to cover, with the foreground colour set to black.

You can create a temporary mask, (known as a quick mask), to help you make a tricky selection. Or you can create a layer mask for more advanced effects. You can move a mask from one layer to another if required, and you can save a mask as a type of channel (alpha channel) if you wish.