Animation is created by displaying a series of images, or cels, in succession.

As each image differs a little slightly from the ones preceding and following it, the variations between them create the illusion of motion when played sequentially.

In traditional animation, cels are done by hand, though in recent years, the animation industry has steadily been moving towards computer-assisted animation. While the concepts used to create the animation (and the end result) are essentially the same, the methods used differ greatly.

The key defining element of all animation, and of "moving" pictures in general, is the number of cels used to create the animation. The more cels the animation contains, the smoother the animated motion of objects appear to be.

In traditional animation, consideration for the number of cels concerns only the total time length of the animation. In computer animation, however, the number of cels contributes to the final size of the animation file, which is a prime concern for files on the Web.

When creating animation for the Web, a balance must be found between animation quality and file size, both of which are affected by the cel count of the animation. Defining the best trade-off point is never easy, but with careful planning and foresight, you can create captivating animations at the least possible size.

The most common form of animation on the Web is made in an image file format known as GIF.