Ajax isnt a technology. Its really several technologies, each flourishing in its own right, coming together in powerful new ways. Ajax incorporates:
* standards-based presentation using XHTML and CSS;
* dynamic display and interaction using the Document Object Model;
* data interchange and manipulation using XML and XSLT;
* asynchronous data retrieval using XMLHttpRequest;
It’s basically the creative use of several mature web technologies in combination. Follow the link above to read more in the very informative article by Jesse James Garrett.