What is the difference between Synchronous postback and Asynchronous postback?

The difference between Synchronous and Asynchronous postback is that: 

1. Asynchronous postback renders only the required part of the page and whereas, Synchronous postback renders the entire page for any postback. 

2. Asynchronous postback executes only one postback at a time, that is, if you have two buttons doing asynchronous postback, the actions will be performed one by one; whereas, Synchronous postback executes all the actions at once. 

3. Asynchronous postback only modifies the update panel that raises the postback; whereas, Synchronous postback modifies the entire page.

Synchronous (Classic Web-Application Model)

A synchronous request blocks the client until operation completes i.e. browser is unresponsive. In such case, javascript engine of the browser is blocked.

As you can see in the above image, full page is refreshed at request time and user is blocked until request completes.

Asynchronous (AJAX Web-Application Model)

An asynchronous request doesn’t block the client i.e. browser is responsive. At that time, user can perform another operations also. In such case, javascript engine of the browser is not blocked.

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.