Before understanding AJAX, let’s understand classic web application model and ajax web application model first.
Synchronous (Classic Web-Application Model)
As you can see in the above image, full page is refreshed at request time and user is blocked until request completes.
Let's understand it another way.
Asynchronous (AJAX Web-Application Model)
As you can see in the above image, full page is not refreshed at request time and user gets response from the ajax engine.
Let's try to understand asynchronous communication by the image given below.