shoeman12,
I found this on the Internet using google:
In fact, a key benefit of Windows Vista graphics system is its ability to
scale depending on video card's capabilities. For example, a high end video
card with 256MB or more of video memory and using a WDDM driver will provide
you with the ultimate graphics interface called Aero Glass that sports
translucent windows, and exceptional 3D effects and animations. A moderate
video card with 64--128MB of video memory and using a WDDM driver will
provide you with the a nice graphics interface called Aero Basic, which will
lack some of the fancier effects. A low end video card that has a moderate
amount of video memory, but doesn't have a WDDM driver, will be able to use a
Windows XP Display Driver Model driver (XPDM) and will provide you with a
more simplistic interface called, Classic, which is comparable to what you
see in Windows 2000.
You can do more research by using google to get more of the details.