See previews of your open windows
When you move your mouse pointer to a taskbar button, a small picture appears that shows you a miniature version of the corresponding window. This preview, also called a thumbnail, is especially useful. And if one of your windows has video or animation playing, you'll see it playing in the preview.
Note
• You can see thumbnails only if Aero can run on your computer and you're running a Windows 7 theme.