A thumbnail is a smaller version of a much larger image that is used to help conserve screen space. They are useful for helping users to view multiple images at once. Thumbnails are also commonly used on the Internet, helping the web page viewer see an image without having to download the full-sized image. Thumbnails help save bandwidth and increase the load time for the visitor. The picture is a thumbnail of a computer motherboard, and clicking the image opens the page to the full sized image.
A thumbnail should not be confused with a thumb drive.