AJAX allows you to send and receive data asynchronously without reloading the web page. So it is fast.
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.
Where it is used?
There are too many web applications running on the web that are using ajax technology like gmail, facebook,twitter, google map, youtube etc.