An Overlay is a special kind of Layer.

• If a Layer is an Overlay, it is not a BaseLayer.
• Overlays (unlike BaseLayers) are not mutually exclusive -- OpenLayers can display many Overlays at the same time (on top of eachother) on a given Map?.
• To hide or show Overlays, the user uses the LayerSwitcher.
• An OpenLayers.Layer? is by default an Overlay (not a BaseLayer)
• For instructions on how to configure an existing Layer as an Overlay, see BaseLayersAndOverlays.