Explanation
Local Area: The local area is in the center of the UI Shell where users do their work. It is the mainwork area and typically contains the transaction form with the menus and controls that enable users to be productive.
Controls in, and the content or state of, the local area generally affect the contents of the contextual area.
Main Area: This term designates the combination of the Local Area and the Contextual Area.
Contextual Area: The contextual area is in the right-hand pane of the UI Shell, with controls and contents that generally are affected by controls in, or the content or state of, the local area; although in specific cases the contextual area can also affect the contents of the local area (causing a local-area reload).
References:
http://docs.oracle.com/cd/E36909_01/fusionapps.1111/e15524/ui_impl_uishell.htm