OpenGL is the one of the graphics services in Mac OS X, others include Quartz and QuickTime.
OpenGL is a sophisticated open-standard, three-dimensional rendering system. Developed as an open source project (OPEN Graphics Library) by Silicon Graphics in the 1990s, OpenGL supports advanced 3D rendering. It is of primary interest to game and multimedia developers. It also has a role in scientific visualization. OpenGL replaces QuickDraw 3D.
For a brief explanation of Quartz, see technical document 43141: "Useful Mac OS X Terms: What is Quartz?"
For in depth information on Quartz, please visit the Apple Developer Connection Web site (http://developer.apple.com/quartz/index.html).
For in-depth information on OpenGL, please visit the Apple Developer Connection Web site (http://developer.apple.com/opengl/index.html).