The GraphicalUserInterface (or GUI for short) is the general term for all the things that a typical Mac OS X application presents in its UserInterface: things like windows, buttons, and menus are all considered to be elements of a GraphicalUserInterface.

An OS X application and its GraphicalUserInterface should JustWork, although this is EasierSaidThanDone. Note also that it is easier said that it is “easier said than done” than it is done.