What is Ajax?
Ajax is a set of technologies being used together, just as "LAMP" is a shorthand referring to Linux, Apache, MySQL and PHP. But back to Ajax!
* HTML + CSS for presenting information
* XML, XSLT and the puzzlingly-named XMLHttpRequest object to manipulate data asynchronously with the Web server.
The difference therefore is with the XML, eXtensible Markup Language, side of things, so let's focus on that. But first, if you've checked out and marveled at the drag-and-pan capabilities of Google Maps or the speedy self-contained table sorting capabilities of Gmail, you've seen Ajax in action.
Want to learn more? Check out Ajax Matters, which is one of the primary reference sites for this rethinking of how users interact with Web sites.
My two cents on Ajax? The name may be rather goofy, but the result of applying the methodology is a quantum leap in Web interactivity. If you're a serious Web developer, Ajax is well worth studying and learning.