What Animated GIF is

A type of GIF format that allows a series of static images to be displayed one after another or on top of each other, giving the effect of motion or animation. Since their implementation in Netscape Navigator 2.0, animated GIFs have been one of the easiest ways to put animation and movement on Web pages. Animated GIFs have been around since the introduction of the GIF89a format in 1989, but they didn't appear on the Web at that time because Web browsers initially did not support GIF89a's animation features.

The meaning of Raster graphics

Also known as "bitmapped graphics" these programs store images in the form of patterns of screen pixels. Unlike "draw programs" (vector graphics), these "paint programs" can create detailed patterns of shading that convey an artistic touch, but any attempt to rescale or resize the image may result in unacceptable distortion. Raster-based graphics have become a standard technology and are popularly known by their GIF and JPG formats. Raster graphics use pixel-by-pixel definitions as opposed to vector graphics which use computer algorithms to describe shapes, lines, animation, etc.