Difference between a Postback and a Callback in ASP.NET

ASP.NET was introduced with a mechanism to post an HTTP POST request back to the same page. It’s basically posting a complete page back to server (i.e. sending all of its data) on same page. So, the whole page is refreshed.In order to understand how this postback mechanism works in ASP.NET, follow the simple steps:

• Add a new ASP.NET web form page to a project e.g. WebForm1.aspx.
• View the page code in HTML Source view. You will find something like following screen.

• Look the form line of code. <form id=”form1″ runat=”server”>
  It represents a server-side implementation of form control.

Now just run the application to see WebForm1.aspx page and view its source code. HTML source of the page will display form element as follows:
<form method=”post” action=”WebForm1.aspx” id=”form1″>

You can see that an HTML form element generated with an HTTP method as “POST” and action=”WebForm1.aspx”. So, if a submit button is clicked, the page will postback to itself by default.Following figure will also give you more understanding on ASP.NET Postback.

“A callback is generally a call for execution of a function after another function has completed.”

 But if we try to differentiate it from a postback then we can say: It’s a call made to the server to receive specific data instead of whole page refresh like a postback. In ASP.NET, its achieved using AJAX, that makes a call to server and updating a part of the page with specific data received.