AJAX is one of the hottest new technologies on the web. It is a core technology for the entire Web 2.0 world. AJAX, which is the system which provides interactivity and customizability, is not one single technology. In fact, it may surprise you to learn that Microsoft is the first major implementer of AJAX with it's Exchange mail web portal.
Sun has long been a player in the AJAX world with its Java Script, but now it is releasing a new framework that looks almost as easy to use but far more powerful. Here is an article that compares the major AJAX systems -- including the new Sun announcement.