Doclets are the standard way to extend or modify the behavior of the Java javadoc
Starting with version 1.2 of the Java SDK, the javadoc tool uses a replacable Java class that conforms to the doclet API to generate the output. A standard doclet is provided that generates HTML javadoc information. Sun unofficially provides at least one more for creating Framemaker and PDF documentation, plus a variety of third party doclets are available. Sun maintains an index of doclets here: http://java.sun.com/j2se/javadoc/faq.html#doclets.