-> A postback is a request sent from a client to server from the same page user is already working with.
-> ASP.Net was introduced with a mechanism to post an HTTP POST request back to the same page. It’s basically posting a completed page back to server on same page. So the whole page is refreshed.
-> IsPostBack is a page level property that can be used to determine whether the page is being loaded in response to a client postback, or if it is being loaded and accessed for the first time.