Mastering Ajax, Part 1: Introduction to Ajax

Ajax, which consists of HTML, JavaScript, DHTML, and DOM, is an outstanding approach that helps you transform clunky Web interfaces into interactive Ajax applications. The author, an Ajax expert, demonstrates how these technologies work together -- from an overview to a detailed look -- to make extremely efficient Web development an easy reality. He also unveils the central concepts of Ajax, including the XMLHttpRequest object.f

http://www-128.ibm.com/developerworks/web/library/wa-ajaxintro1.html?ca=dgr-lnxw01MasterAJAX