Getting Checked rows from gridview in asp.net

I have a GridView in ASP.net where I have a CheckBox column. The user can toggle the CheckBox. Now, what I want is that when the user clicks on a button, all the records from the GridView where the CheckBox is checked should be displayed. And on another button, the opposite state should be displayed…
I am not getting the logic for the same.

ASP.NET: Unfamiliar with Interfaces

I’m building a decent sized application in ASP.NET/VB.NET with various objects… I’ve never used interfaces before, and a fellow programmer balked when I mentioned this to him. Can anyone give me a quick overview on how they’re used, what they’re used for, and why I would use them? Maybe I don’t need to use them for this project, but if they would help, I surely would love to try.