Java is a programming language.
Developed in the years 1991 to 1994 by Sun Microsystems.
Programs written in Java are called applets.
The first browser that could show applets was introduced in 1994, as "WebRunner" - later known as "The HotJava Browser".
You do not need to know Java to install applets on your pages. There are thousands of free applets available on the internet for almost any purpose. Most of them can be customized without programming.
Most of today's browsers can run applets. The ones that can't, aren't of much importance, since very few users have such outdated browsers. To be more precise, applets can be embedded in pages viewed by Netscape 2+ and Internet Explorer 3+.
However, some people have turned off the ability to run applets in their browser. This is in most cases companies with more or less paranoid ideas of potential hacking.
No matter what their motivation is, it's a fact that there are a minor amount of people out there, that will not see your applets, even if their browser is capable of showing it. This should be taken into consideration before deciding to add applets to your pages.
An applet can be embedded into a webpage. Usually the applet has several settings that will allow you to personalize it. For instance, if you insert an applet that will work as a menu, you can specify which options should be in the menu, and which pages should be loaded upon click on an option.
Since Java is a real programming language there aren't many limitations to it. Any program running on your computer could possibly have been made as an applet. Spreadsheets, wordprocessors, graphics programs... even entire browsers could be made with Java. However, most applets used on webpages serve much smaller purposes than the ones mentioned. The reason is simple: They need to be transferred through the internet, and therefore can't take up just any amount of space.