Working with masks and channels
When you want to edit a specific area of a layer, it can be difficult to keep edits from going past the precise edge you want. Or you might want to apply an adjustment layer or a filter to only part of a layer. Both are easier using a mask, a type of image overlay that controls which layer areas will be affected by edits.
You can also use a layer mask to mark transparent areas of a layer. Layer masks are essential for removing backgrounds, combining multiple images into a composite image, and controlling which areas are affected by adjustment layers. A layer mask is more useful than deleting unwanted areas of a layer, because it’s reversible: By painting in a layer mask, you can make hidden areas visible again.
In addition to color channels, an image can contain alpha channels, useful for saving and reusing selections and marking transparent areas for an entire document.
It can be challenging to paint or draw a mask or channel edge that precisely follows a nonsharp border of a subject, such as fuzzy hair. Photoshop provides tools that use automation to help you quickly create complicated selections and masks.