O'Reilly's Mac DevCenter has an article on MacOS X Quartz by Matthew Russell. Here's an excerpt:
Quartz is the heart and soul of Mac OS X's graphics layer, which directly supports the defining features of the Aqua desktop experience. Quartz is largely based upon Adobe's PDF specification, but it has roots tracing all the way back to PostScript. The two defining components of Quartz are Quartz Compositor and Quartz 2D. Quartz Compositor is OS X's powerful window server, and Quartz 2D is the two-dimensional drawing engine that's often referred to as Core Graphics. Although Quartz 2D is accessible through the Application Services umbrella framework, Tiger introduced Quartz Composer: an alternative way to explore the power of Quartz through a powerful visual programming environment.