Bucky Roberts of the New Boston gives the best explaination of AJAX out there on the web. "see video here"
AJAX stands for Asynchronous JavaScript and XML. AJAX is a new technique for creating better, faster, and more interactive web applications with the help of XML, HTML, CSS, and Java Script. Ajax uses XHTML for content, CSS for presentation, along with Document Object Model and JavaScript for dynamic content display.
AJAX allows you to update parts of a web page and not the entire page. It makes pages sleeker and faster and gives more of an interactive feeling to the page
It also allows for communication through XML and JSON's to the server to quickly update information in specified fields
AJAX gives partiality to a page and enhances the communication aspects through Asynchronous communication