In other words, Ajax allows web pages to be updated asynchronously by exchanging small amounts of data with the server behind the scenes. This means that it is possible to update parts of a web page, without reloading the whole page.
Examples of applications using AJAX: Google Maps, Gmail, Youtube, and Facebook tabs.
Why Use AJAX?
The user will stay on the same page, and he will not notice that scripts request pages, or send data to a server in the background. By using the XMLHttpRequest object, a web developer can update a page with data from the server after the page has loaded. The XMLHttpRequest object is supported in Internet Explorer 5.0+, Safari 1.2, Mozilla 1.0 / Firefox, Opera 8+, and Netscape 7.
How Ajax Work?