It isn't some new technology but it is a new way to use existing methods to build rich, user-friendly, desktop style of interacting with a web page.
A good example of an AJAX application is Digerati's Shopping Cart build into myCommerce for Goodbuddy Sports website. Notice the page doesn't 'reload' with information but appears to be working in the background? This is pro, it becomes RICH.
Technologies it uses are:
- Dynamic Server Side Technology (ASP.net, ASP, PHP, JSP etc.)