Postback is a propery in which it is alerted to the
webserver. For a instance if there is a submit button, and
when the user click the button the particular page will not
be sent to the webserver which was triggered by the user.
Instead of that, the postback propery ensures an alert
message to the webserver that if the submit button is
clicked the page should be fired.

by default controls on microsoft asp.net web pages tat 
causes a postback such as the button control,postback to 
same page for processing.
how ever you can configure  cntrols to post to a different 
page within the same application,to do tis use the postback 
URL  property  of the control to specify the page to which 
the control should post