In Asp.Net, By default the button control posts back to the same page that contain the button. In most of the cases this behavior is ok. But sometime we want to be able to post to another page in the application. In Asp.Net 1.X we used the Server.Transfer method to move between the pages without changing the URL. 

But in Asp.Net 2.0 we can post to a different page, firing a normal post back to a different page inside the application. We can also access the values of the server control of the source page that initiated the post back.

Cross page post back can be used by control that implements IButton Interface (Button, Link Button, and Image Button). We need to set the PostBackUrl property to specify the target post back page on the click event.

To access the previous page property in the target page we use the PreviousPage property. By default the previousPage property is of page type hence we need to access the control using the FindControl method. You can also enable strongly-typed access to the source page by setting the @PreviousPageType directive in the target page to the virtual path or Type name of the source page. If you intend for multiple pages to post to a single target page, you cannot use the strongly-typed directive.

Hope this helps
Thanks
Vikram