Source : Free On-Line Dictionary of Computing

Animated GIF
     
         (GIF89a) A variant of the {GIF}
        {image} format, often used on {World-Wide Web} pages to
        provide moving {icons} and banners.
     
        The GIF89a format supports multiple "frames" that give the
        impression of motion when displayed in sequence, much like a
        flip book.  The animation may repeat continuously or play
        once.
     
        Animated GIFs aren't supported by earlier {web browsers},
        however the first frame of the image is still shown.
     
        There are many utilities to create animated GIFs from a
        sequence of individual GIF files.  There are also utilities
        that will produce animated GIFs automatically from a piece of
        text or a single image.
     
        One problem with this format is the size of the files
        produced, as they are by definition a sequence of individual
        images.  Apart from minimising the number of frames, the best
        way to decrease file size is to assist the {LZW} compression
        by using blocks of solid colour, avoid {dithering}, and use
        fewer colours.  If areas of an image don't change from one
        frame to another, they don't need to be redrawn so make the
        area a transparent block in the second frame.
     
        (1999-08-01)