The result is a back-and-forth that usually involves clicking a button, waiting for the server, clicking another button, and then waiting some more. With Ajax and the XMLHttpRequest object, you can use a request/response model that never leaves users waiting for a server to respond. In this article, Brett McLaughlin shows you how to create XMLHttpRequest instances in a cross-browser way, construct and send requests, and respond to the server.
http://www-128.ibm.com/developerworks/java/library/wa-ajaxintro2/index.html?ca=dgr-wikiAJAXinto2