transparent GIF 
A feature of the GIF89a graphics standard, a transparent GIF lets the background show through selected parts of an image. When creating a GIF, the designer can designate one color in the image's palette to be transparent. When the GIF is displayed, areas using that color reveal whatever is underneath. Transparency is most often applied to a GIF's background color to let a Web page's background show through, so that images appear to float on the page. Most modern Web browsers support transparent GIFs. Those that don't simply display the images as normal GIFs.