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.