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.
When these technologies are combined in the AJAX model, web applications are able to make quick, incremental updates to the user interface without reloading the entire browser page. This makes the application faster and more responsive to user actions.
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.