Layers (from MPEG-4) are zones of the 3d window containing 2D or 3D contents, displayed above the main scene. They are particulary useful for creating user interfaces (note that when you move, you move only in the main scene: you have to modify the viewpoint of the layers' parameters to change their view).

As you can see in the example, the fog, background, viewpoint, etc.. are individual (however you can re-use some elements between layers, or have layers in layers). When there is no fog nor background defined in a layer, it merges in its parent.