A common goal of portal developers is to be able to build portlets that render independently of other content on the portal desktop. That way, the portlets are in effect mini-Web applications with which users can interact without having to refresh the entire portal desktop. Asynchronous JavaScript and XML (AJAX), a newly emerging technology, can help you achieve this goal.

Website: http://developers.sun.com/prodtech/portalserver/reference/techart/asynch_rendering.html