About PostBack - ablealias/asp.net GitHub Wiki

What is PostBack?

Sending data back to the Web server as part of user request is often referred to as PostBack in ASP.Net. An Asp.Net Web page contains a property called IsPostBack that is used to determine if data is being sent back to the Web server or if the Web page is simply being requested.

For example, if you want to login a Web site, you need to access a web page to provide login credentials such as username and password, suppose you're going to access the web page login.aspx, in this scenario first time you access the web page login.aspx PostBack shouldn't happen because you're not sending data back to the web server, you just get the web page login.aspx, but when you give the username and password and click submit button then the form sending data back to the server and PostBack happening.

IsPostBack is a property of Asp.Net page that tells whether or not the page is on its initial load or if a user has perform a data submit on the Web page and that has caused the page to PostBack to itself. IsPostBack property returns TRUE if the page is being loaded in response to a client PostBack otherwise return FALSE.