I am quite new to ajax but love it already. We have surprised our customers with Ajax so far.
A brief definition is:
A scripting technique for silently loading new data from the server. Although AJAX scripts commonly use the soon to be standardized XMLHttpRequest object, they could also use a hidden iframe or frame. An AJAX script is useless by itself. It also requires a DOM Scripting component to embed the received data in the document.
domscripting.webstandards.org/
Mainly, ajax is the combination of JavaScript/DHTML, DOM and the XMLHttpRequest. It is not a protocol or language, just a technique that allows you to perform asynchronous requests from the Browser to the Server in order to dynamically update the page without a whole page refresh.
However, Ajax is just a client side thing ... you can use any server side technology withit.
Hope this helps