Recommendations for Abstract Classes vs. Interfaces: Visual Basic and Visual C# Concepts [source: MSDN]

On abstract classes: “An abstract class is a class that cannot be instantiated, but must be inherited from. An abstract class may be fully implemented, but is more usually partially implemented or not implemented at all, thereby encapsulating common functionality for inherited classes.”

On interfaces: “An interface, by contrast, is a totally abstract set of members that can be thought of as defining a contract for conduct. The implementation of an interface is left completely to the developer.”