AT THE MOST BASIC LEVEL OF IMAGE EDITING, you edit the pixels of an image. That's a straightforward way of working, but once you change the pixels, the ability to change your mind and keep your options open is limited. When you create a document that consists of discrete compositional elements, layers provide a way to maintain the independence of each element in a composition. Layers are like a stack of clear film with a different graphic on each piece of film. If you frequently combine multiple elements into a larger image, such as a digital collage, layers let you rearrange elements at any time without permanently erasing what's underneath. Layers also allow other kinds of data, such as type and smooth vector paths, to exist in a Photoshop document with pixel-based (bitmap) layers.