Quartz Extreme
From Applepedia
Basically the idea behind Quartz is that OS X is using your graphics card to render the user interface, which leaves your processor free to do more useful things.
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Description from Apple
Every time you move or resize a window on your Mac, Quartz uses the integrated OpenGL technology to convert each window into a texture, then sends it to the graphics card to render on screen. The graphics processor focuses on what it does best — graphics — freeing the PowerPC chip to perform more operations in the same amount of time. Drawing windows, resizing, moving, scrolling — everything is zippier.
