How to: Specify Whether ASP.NET Web Server Controls Post to the Server
By default, information and selections that the user specifies in an ASP.NET Web page are not posted immediately to the server. Instead, the values are posted only when the entire page is posted, usually in response to clicking a button. However, you might find it useful at times to send user changes to the server immediately.
To post control values to the server immediately
Set the control's AutoPostBack property to true.
You can also respond to user actions by creating client scripts. For example, you can create a client script to respond to the Click event of a button or check box. For more information, see Client Script in ASP.NET Web Pages