Prior to AJAX, any retrieval of data from the server required that the entire Web page be refreshed in the user's computer. As a result, systems were often designed with less interaction; for example, to have the user submit a form only at the end, after all fields were filled in. In contrast, AJAX systems can validate one or two items at a time "behind the scenes" without making the session cumbersome, especially over slow connections. AJAX, also known as "remote scripting," allows Web-based applications to be designed like LAN-based applications.
The XMLHTTPRequest Object