XMLHttpRequest object

XMLHttpRequest object

Suppose if we have to develop a user registration page. If we use traditional JavaScript method after the user fills the form he or she will have to click the submit button and after that request will be processed and a new page will be loaded accordingly. But if we use AJAX upon submission the user stays on the same page and transfer of data takes place in the background. It’s much more user-friendly and also you can provide some additional features such as username suggestions, checking username availability and all this can be done automatically when user enters data without reloading the page.
When using AJAX, XMLHttpRequest is used by JavaScript for communicating directly with the web server.

http://www.botskool.com/programming-tutorials/ajax-tutorials/ajax-basic-tutorial