In Windows 7, you can point to a taskbar button to see a live preview of its open windows—including webpages and live video. Move your mouse over a thumbnail to preview the window full screen, and click it to go open the window. You can even close windows and pause video and songs from the thumbnail previews—a big time saver.
Live taskbar previews are available only in the Home Premium, Professional, Ultimate, and Enterprise editions of Windows 7.
The new taskbar previews show you what's happening on your desktop.