There are two kinds - color and selection. You can have up to 24 of them in an image .
Masks are like stencils that protect parts of an image.
An RGB image has four color channels - red, green, blue and a composite RGB channel. You can perform color corrections on individual channels for precise adjustments. These contain selection information. When you make a selection using one of the selection tools, the area within the selection is editable and the area outside is protected - or masked. You can store selections as alpha channel where they will be saved for use again in the future.