Java Webstart is very useful for automatic updates of an application over
the internet. Every time you click on an webstart link, Java Webstart
checks if the program was updated since your last use. If this is the case,
webstart will download and execute the program. If not, webstart will use a
Java Webstart is included in the Java Runtime Environment from Sun Microsystems. You need to download and install it to use a java program anyway.
Make sure your browser runs Java Webstart for the MIME type application/x-java-jnlp-file. To setup your browser to do this, you have to add a new MIME type in your browser settings. The MIME type has to be application/x-java-jnlp-file, extention jnlp and the application path should point to zour Java Webstart installation (usually something like c:\program files\java web start\javaws.exe).
Java Webstart configuration for Opera