On the QaFoo.com blog Kore Nordmann shares some insight about the difference between abstract classes and interfaces and how they can be used effectively/correctly.

Features of object oriented languages are often use from a purely technical perspective, without respect to their actual semantics. This is fine as long as it works for you, but might lead to problems in the long run. In this article I discuss the semantical differences between abstract classes and interfaces. I also outline why following the semantics of those language constructs can lead to better code.

He defines "class" and "interface" before he gets too far into the differences, stating that a "class" is more of a "type" while an "interface" describes the type and the structure to use it. He also talks about how to tell them apart (when to use each). There's also some examples and "hints" to help you determine good and bad examples of interfaces and when an abstract class might be a better answer.