Layers are used in digital image editing to separate different elements of an image. A layer can be compared to a transparency on which imaging effects or images are applied and placed over or under an image. Today they are an integral feature of image editors.
The standard kind of layer is called simply “Layer” in most programs. It contains just a picture which can be superimposed on another one. The picture can cover the same area as the resulting picture, just a part of it, or, in some cases, a bigger part than the final picture.
A layer can have a certain transparency/opacity and a number of other properties. In a high-end program like Adobe Photoshop, a basic layer may have more than a hundred different possible settings. Even though some of them overlap and give the same result, they give a skilled user a lot of flexibility.