what is Ajax?

AJAX stands for Asynchronous JavaScript and XML.
AJAX is the way of exchanging data with a server, and updating parts of a web page – without reloading the whole page.
Example:- Ajax is used many web application like google, facebook, yahoo, twitter, youtube etc.

Why use Ajax?
We will explain through example, Traditionally webpages required reloading to update their content. For web-based email this meant that users had to manually reload their inbox to check and see if they had new mail. This had huge drawbacks: it was slow and it required user input. When the user reloaded their inbox, the server had to reconstruct the entire web page and resend all of the HTML, CSS, JavaScript, as well as the user’s email. This was hugely inefficient. Ideally, the server should only have to send the user’s new messages, not the entire page through Ajax.

Difference between Synchronous and Asynchronous?
Synchronous means that data is sent in a sequence and Asynchronous means that data is not sent in a sequence.
Synchronous waits for each operation to complete, after that only it executes the next operation. Asynchronous never waits for each operation to complete, rather it executes all operations in the first GO only.