Render passes are a mechanism for dividing a scene into several "layers", each of which isolates different scene attributes, such as shadows, lighting components, ambient occlusion, mattes, and so on. Though a number of preset passes are available, you can create custom passes to render exactly the information you need.
The beauty pass (center) surounded (from left to right) by its background pass, foreground pass, matte pass, shadow pass, and highlight pass.
