From Mac Guides
Core Animation is a feature of Mac OS X 10.5 (Leopard). It allows developers to add animation features to their applications, with support for thousands of layers (text, images, video, OpenGL etc), color, size, opacity, and more. The developer needs only to specify the start and the end, and Core Animation handles the rest.
Other more discreet uses of Core Animation are also seen in OS X's default Web Browser, Safari with the way that tabs dragged out of the tab bar expand into a new Window.