The difference between synchronous and asynchronous postback is as follows:

Asynchronous postback renders only the required part of the page; whereas, synchronous postback renders the entire page for any postback.

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.

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