It's a cache of thumbnail images of all of the pictures in the directory. Windows uses it for various photo-gallery views.

Thumbs.db is the thumbnail cache for the whatever pictures/movies you have in the folder. It helps Windows to display thumbnails quicker in explorer when viewing the contents of the folder. When you delete the file Windows creates new thumbnails and stores them in this file to load them quicker next time.

Despite the fact that Thumbs.db is a hidden system file, it annoys people because it tends to show up in places where it’s not wanted. Many times you’ll try to zip up a folder full of images and e-mail them to a friend or upload them to an FTP site, and Thumbs.db goes along for the ride.

To stop your computer from generating and regenerating future Thumbs.db files, do the following:

If you’re on the desktop…

1. Click Start
2. Double-click Control Panel
3. Double-click Folder Options

Or, if you have My Computer open and are browsing any folder in your system…

1. Click Tools (next to File, Edit, View at the top of the screen)
2. Click Folder Options

After performing either of those two operations, the “Folder Options” window will open up.

1. Click on the View tab
2. Check off the circle next to Do not cache thumbnails
3. Click the Ok button

Once you click the Ok button, your computer will cease to generate Thumbs.db files. If you delete any of the existing Thumbs.db files, they will not return. Be forewarned though, if you browse a folder that contains a large quantity of image files (or extremely large image files), it will take a long time for that folder to load even if you have previously browsed it because the thumbnail images will not have been cached in Thumbs.db.