About masks (Photoshop)
Masks let you isolate and protect areas of an image as you apply color changes, filters, or other effects to the rest of the image. When you select part of an image, the area that is not selected is "masked" or protected from editing. You can also use masks for complex image editing such as gradually applying color or filter effects to an image.
Masks and channels are grayscale images, so you can edit them like any other image. With masks and channels, areas painted black are protected, and areas painted white are editable. Masks let you save and reuse time-consuming selections as alpha channels. Alpha channels can store selections so you can use them again, or you can load a saved selection into another image.
Photoshop lets you create masks in the following ways:
Quick Mask mode
Lets you edit any selection as a mask. The advantage of editing your selection as a mask is that you can use almost any Photoshop tool or filter to modify the mask. For example, if you create a rectangular selection with the Marquee tool, you can enter Quick Mask mode and use the paintbrush to extend or contract the selection, or you can use a filter to distort the edges of the selection. You can also use selection tools, because the quick mask is not a selection. (See Using Quick Mask mode to make selections (Photoshop).) You can also save and load selections you make using Quick Mask mode in Alpha channels (see Saving a mask selection).
Alpha channels
Lets you save and load selections. (See Storing masks in alpha channels.) You can edit Alpha channels using any of the editing tools. When a channel is selected in the Channels palette, foreground and background colors appear as grayscale values. (See Using Quick Mask mode to make selections (Photoshop).)
Layer masks and vector masks let you produce a mix of soft and hard masking edges on the same layer. By making changes to the layer mask or the vector masks, you can apply a variety of special effects. (See Masking layers.)