Java runtime lets you run programs made with the Java programming language. Java programs usually run in applets in a web browser, so users can play Java games, or run programs like calculators in their web browsers.
Java programs also can be run from your desktop in an Applet window.
To install Java runtime plugin for your web browsers in Ubuntu 9.10, open up Ubuntu Software Center which can be found at the bottom of the applications menu.
Type in Java in the text box on Ubuntu Software center and Java should be at the top of the list. Now you can install it by clicking the install button.