There are a number of ways to present Mathematica animations on the web. Unfortunately, Mathematica does not automatically convert animations into a proper format. On the Macintosh, animations can be exported as QuickTime movies that can be viewed with Apple's free viewer. On all platforms, the Export command supports exporting animations created in Mathematica as animated GIFs. Any list of graphics passed to this will be exported as an animated GIF file.
Export supports a large number of options, including animation speed and color reduction.
Please refer to the documentation about Export for more information on these options. Export could not handle animated GIFs in Mathematica 3. Also, on some platforms, exporting a set of graphics as an animated GIF file takes a very long time in Mathematica 4.0. This is due to a specific implementation of the export routines that will be changed in future releases. Right now, there is a slightly more inconvenient, but about 25 times faster, alternative to using Export that also works in Mathematica 3.