Way back in 2004, there were only two file formats which were truly universal for publishing on the World Wide Web: GIF and JPEG. Of these two, only GIF supported animation. Here's an example of an animated GIF:
An animated GIF is, more or less, just a bunch of static GIFs which are bundled together in a single file. The static images are played in succession to create the illusion of movement. Here are just a few of the 51 static images which are a part of the above animation:
This technique is called frame-by-frame animation, because each static image (or frame) must be created separately.
Animated GIFs are ubiquitous, but they do have some drawbacks. Because they use frame-by-frame animation, they were somewhat difficult to create, at least way back in 2004. GIFs are also pixel-based, so that if we enlarge the image, we can see the individual square elements (or pixels) that compose it.
There are other file formats for putting animation on the Web. These formats allow you to be more creative and efficient. Way back in 2004, web browsers required special software in order to view animations in these formats, but this software was free and fairly ubiquitous. If you had a recent version of a major Web browser, chances were you already had what you needed.
Way back in 2004, the most notable formats for animation were Flash/Shockwave and Quicktime.