Quartz

Quartz is the graphics layer that sits on top of the Darwin core of Mac OS X, sometimes also referred to as Core Graphics. Quartz directly supports Aqua by displaying 2D computer graphics to create the user interface, including on-the-fly rendering and anti-aliasing with sub-pixel precision.

There are two components that make up Quartz:
 * Quartz Compositor - A compositing windowing system that manages and composites off-screen window bitmaps to create the Mac OS X user interface.
 * Quartz 2D - A graphics library which uses the PDF document format to draw two-dimensional text and graphics.

Articles

 * Mac OS X Update: Quartz & Aqua by John Siracusa at Ars Technica
 * Why Apple didn't use X for the window system by Mike Paquette at Slashdot (2003-08-19)