AJAX, an acronym for Asynchronous JavaScript and XML, is a group of technologies combines to accomplish powerful new forms of the web interactions. It’s composed of JavaScript, the XMLHTTPRequest object and XML, JSON, (X)HTML or even plain text.

Asynchronous means that you can make an HTTP request to a server and continue to process other data while waiting for the server’s response. For example, you can make requests to a server-side script to retrieve data from a database as XML, send data to a server-side script to be stored in a database or simple load a static XML file to populate pages of your web site without ever refreshing the page.

Behind the scenes data transfers are accomplished through the XMLHTTPRequest object, which is the heart of the AJAX engine, as it’s used to exchange data asynchronously with a server-side language, XML or other text format. It combines with the DOM (Document Object Model) to display the AJAX response data as (X)HTML and CSS.

The DOM is a language-independent interface that makes common web page elements accessible through