Some browsers display interlaced GIFs as they read them, first they display every eight line, then fourth line, then every second line and then every line. This display method makes people, waiting for the image to load, to quickly figure out what the final image would look like.
Other browsers bring in a rough version of the graphic and gradually refine it. GIFs also support animation. It is possible to piece together numerous GIFs and create a simple movie.