Layer
A stacked element in an image document that can hold pixels, adjustments or effects independently of the layers below it.
Measured in
count
In depth
Layers separate an image into components that can be moved, masked, blended and reordered without affecting one another. The stack is evaluated from bottom to top, so order matters: a curve above a retouching layer applies to the retouched result, while the same curve below it does not.
Examples in practice
Placing a retouching layer above the background lets every clone stroke be erased later without touching the original pixels.