Synchronous postback:
1. It renders the entire page for any postback from the web server
2. It executes all the actions at once because page requests at a time to web server
3. It modifies the entire page, same as the general postback
Asynchronous postback:
1. It renders only the required part of the page from the web server
2. It executes only one postback at a time
3. It only modifies the update panel that raises the postback