Aero Glass and the New UI
Have you ever had a window on your Windows XP desktop (or any other version of Windows) that was busy, so you move it out of the way to see the window beneath it, and you end up with a big empty rectangle where the busy window used to be? That's the unfortunate artifact of a desktop drawing system that is simply years out of date. Vista gets rid of that, and does so much more, by totally changing the entire way the screen is drawn.
If you've got a DirectX 9 graphics card with 128MB of RAM or more, you'll be able to enable the "Aero Glass" desktop in Vista. This is the real Vista desktop, and an old version that works like the current Windows XP GDI+ desktop drawing system exists in Vista only for backwards compatibility with systems that don't have the graphics hardware required for Aero Glass.
Why the need for a reasonable DX9 graphics card? Vista includes a new desktop compositing and drawing system that uses DirectX 9 to draw the screen. Every window, icon, toolbar, or other desktop element is actually a 3D surface, made of polygons and manipulated by your graphics card. It's possible to smoothly stretch, rotate, skew, light or shadow, and otherwise manipulate everything on the desktop using all the flexibility of DX9. Everything gets rendered to an off-screen buffer, and then swapped to the live desktop view. This enables all kinds of cool effects, like windows that can warp and stretch, but that's just eye candy.