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