Ajax is Asynchronous JavaScript and XML and first got web developers attention when Google Maps was released. Google Maps being the first really cool application anyone had done with Ajax.

Ajax is nice in that much of the information exchange between the user and the webserver happens in the background and entire page refreshes are not needed to update information.

While Ajax claims to be cross-platform working the same on all platforms you’ll find in practice you’ll likely need to make various tweaks for different browsers.

W3 Schools also has an Ajax tutorial