The Preview Pane is a nifty feature of Windows 7 that lets you visualize the file that is currently selected directly in the explorer window. Although previous versions of Windows had some kind of possibility to preview files, Microsoft seems to have done it right with Windows 7.
The user sees previews of images, audio files, video files and documents. Other types of files can also be added. This feature in useful in many situations because you can see the contents of a file without having the need to open a specialized application for that file and this saves you a lot of time.
You can activate the preview pane by clicking on Show the preview pane button which is located on the top right side of the window.
After the preview pane appears you can resize it if you want to by clicking and holding its left margin and moving it to the left or to the right.
If you want to disable the Preview Pane you can use the same button you used to open it.