I believe that the official sources gives a much better introduction to Quartz Composer then me coming up with something on my own. So here they are.
From http://developers.apple.com …
“Quartz Composer is a groundbreaking graphics development environment that allows you to explore the incredible power of the graphics stack of Mac OS X Tiger. With Quartz Composer, you can easily combine the capabilities of Cocoa, Quartz 2D, Core Image, OpenGL, and QuickTime, all using an approachable visual programming paradigm. Use Quartz Composer to prototype Core Image filters, build engaging screen savers, create custom user-interface widgets, make data-driven visual effects, and even perform live performance animations.”
From http://en.wikipedia.org/wiki/Quartz_Composer …
“Quartz Composer is a node based visual programming language provided as part of the Xcode development environment in Mac OS X v10.4 “Tiger” for processing and rendering graphical data. Quartz Composer uses OpenGL, Core Image, Core Video, and other technologies to build an API and a developer tool around a simple visual programming paradigm. Apple has embedded Quartz technologies deeply into the operating system. Compositions created in Quartz Composer can be played standalone in any QuickTime aware application (although only on Mac OS X 10.4 and later), from inside the Quartz Composer application, or can be embedded into a Cocoa or Carbon application. Because Quartz Composer makes extensive use of hardware acceleration and pixel shaders, it is recommended to have a recent graphics card with at least 32MB of VRAM.”