9.2. The Window Server and Quartz

The Mac OS X Window Server manages the screen (or screens), the keyboard, and the mouse. Unlike the X Window Server and the NeXTSTEP Display PostScript Window Server, the Mac OS X Window Server is lightweight — it doesn't actually do the drawing itself. Instead, its primary role is to manage which regions of the screen each application is allowed to use, and to allow the applications to do their own drawing. The main job of the Window Server, then, is to assure that programs draw in their own rectangular piece of real estate on the screen, and to see to it that events destined for one program aren't accidentally sent to another.

The Window Server frees you from having to worry about interactions between your program and other programs that are running simultaneously. For all intents and purposes, you can design your program as if it is the only one running.