AJAX meant to increase the web page's interactivity, speed, and usability. AJAX is not a new programming language, but a new way to use existing standards. AJAX is the art of exchanging data with a server, and updating parts of a web page - without reloading the whole page.
AJAX allows you to send only important information to the server not the entire page. So only valuable data from the client side is routed to the server side. It makes your application interactive and faster.
AJAX is a web browser technology independent of web server software.
A user can continue to use the application while the client program requests information from the server in the background.
Intuitive and natural user interaction. No clicking required only Mouse movement is a sufficient event trigger.
Data-driven as opposed to page-driven.
There are many web applications now a days using Ajax technologies in the web application as like gmail, facebook, twiter, etc.
Contents for AJAX
Synchronous and Async in AJAX