AJAX is a web technology for creating fast and dynamic web pages.
AJAX allows web pages to be updated asynchronously by exchanging small amounts of data without refreshing the whole page. This means that it is possible to update parts of a web page, without reloading the whole page.
Web pages that do not use AJAX must reload the entire page if the content change.
Examples of applications using AJAX: Google Maps, Gmail, YouTube, and Facebook tabs.
AJAX is based on Internet Standards
AJAX is based on internet standards, and uses a combination of:
XMLHttpRequest object (to exchange data asynchronously with a server)
CSS (to style the data)
XML (often used as the format for transferring data)