GIF89a Animations are a feature available only with Netscape 2.0 or better, and to a limited extent with Microsoft's Internet Explorer. A bandwidth-friendly alternative to CGI or Java based animations, they are in many respects superior to their competitors, as they need only be downloaded once, after which they play from cache.
Any animation can be converted into a GIF animation. Quicktime movies, AVI files, or MPEGs can also be translated into this form. Among the advantages are the fact that you do not have to wait for a whole file to download, you'll get slow motion, streaming video on the first pass, and full speed animations thereafter.