Arox would reply to you with unfunny picture or something, so here's a wikipedia definition for you: http://en.wikipedia.org/wiki/GIF
The Graphics Interchange Format (GIF; /ˈdʒɪf/ or /ˈɡɪf/) is a bitmap image format that was introduced by CompuServe in 1987 and has since come into widespread usage on the World Wide Web due to its wide support and portability.