Web Design in a Nutshell, 3rd Edition

How They Work

Animated GIFs work a lot like traditional cell animation. The file contains a number of frames layered on top of each other. In simple animations, each frame is a complete scene. In more sophisticated animations, the first frame provides the background and subsequent frames just provide the changing portion of the image.

A GIF animation file consists of a number of images and a set of instructions that specify the length of delay between frames, as well as other attributes like transparency and palettes.