AJAX – Mozilla Developer Center
Asynchronous JavaScript and XML (AJAX) is not a technology in itself, but is a term that describes a “new” approach to using a number of existing technologies together, including: HTML or XHTML, Cascading Style Sheets, JavaScript, The Document Object Model, XML, XSLT, and the XMLHttpRequest object.
This is a very handy site for all things AJAX, and as an added bonus…this site in general, Mozilla Developer Center has a ton of information and links to all things a web developer could want. 
Wandering the Interwebs 