Question: I want the effect that looks like the screensaver where there are stars moving across your screen. What is it?

Many people make the mistake of thinking that this is done with an animated gif. It isn't. It's done with a java applet. There are many online sites, listed below, where you can download the applet that will allow you to achieve this affect on your site. Among them are:
 

Java Powered
Has the best "starfield applets", in Webdesigner-Freebies' humble opinion.