AJAX stands for Aysnchronous Javascript And XML. It is just a framework of grouping and using several well known technologies. Javascript is the foundation of this framework. There is actually no new technology around AJAX.

Its popularity is started by Google, and its "no new education" for developers. The only pitfall about AJAX is that it is too easy to gather information "under the hood" without user knowledge. Advertisers love it because it gathers a lot of consumers habbit. Users should hate it to some extent (obviously)!