These are the elements that you can render as separate images.
Here is a rendering of a fountain, against a checkered background, and various elements.
On the right is the fully rendered fountain. On the left, from top to bottom, are diffuse, specular, shadow, and reflection elements.
Two more elements not directly related to the objects in the scene, but important when compositing the image to other sources: On the left is the background, on the right is the alpha channel.
On the left is an atmosphere element, in this example, a light fog on the back side of the fountain. On the right is the Z-depth. The fog uses the depth of the image and objects to determine its density. The Z-depth element contains these depth values.