Java Plug-in software enables enterprise customers to direct applets or JavaBeans on their intranet web pages to run using Sun's Java 2 Runtime Environment, Standard Edition (JRE) instead of the web browser's default virtual machine. Sun's JRE provides a Java Compatible environment for today's widely adopted web browsers. That means consistency and reliability when running applets.
Java Plug-in 1.3.1 Documentation
Java Plug-in documentation for 1.3.1 is currently the same as for the 1.3 release. Note,however, that the HTML Converter is now bundled with the Java 2 SDK, which contains Java Plug-in.