To use Windows Flip, press ALT + TAB as you would do in Windows XP or previous versions of Windows - this displays a similar alt-tab system except with aero enabled you can see preview thumbnails of applications that are open whih are updated in real time.

To use Flip 3D press the Windos Key+TAB : This is a feature new to Windows Vista, this requires aero to be enabled. This is much more useful than Windows Flip as you can quickly flip through all your running applications and see whats going on with each one quickly and in a much larger preview than the thumbnails from Windows Flip.