In addition to the reference by Griff, please read this discussion:
When we use abstract and when we use interface...?.
[EDIT]
One more important thing: in .NET structures cannot inherit from non-interfaces. They can implement muliple interfaces though. This is practically the only OOP feature applied to structures (and value types in general). Most developers did not pay attention to this important feature (unfortunatly, including myself, until someone brought my attention to it).
—SA