What exactly is Quartz Extreme???

I read somewhere that Quartz Extreme is an API, so that means that it is the Apple equivalent of OpenGL and Direct X? So in everyday life when is Quartz working? Are there any guides or tips of fooling around in the Quartz Compoz]ser? I know that there is a node based editor for making screensavers, just wondering are there any tips and trick to play with it.