Java Applets (see a load to try/use on
http://www.freewarejava.com/ ) are a multi-platform alternative to Microsoft's proprietary ActiveX technology.
If you follow the MozillaZine forums - you may notice a heated debate about giving Mozilla an ActiveX plugin - but that's beside the point.
JAVA (no real relation to "javascript") is a programming language geared toward web trinkets
Anyway, Microsoft ship a very OLD version of JAVA - 1.1, and have a poor relationship with SUN, developers of Java (now Java 2 1.41).
IBM's version of Java (currently Java 2 1.3) is tuned for their "websphere" technology, and is often reckoned to be better optimized - unfortunately, some interestiong stuff (such as the "arachnophobia" web editor
http://www.arachnoid.com/index.html
Are beginning to need J2SE 1.4 (That's "Java 2 standard edition, version 1.4")
People are screaming for the IBM supertuned version 1.4 to arrive!