What is the difference between an interface and abstract class?

Question

What exactly is the difference between an interface and abstract class?

Answer 1

Not really the answer to the original question, but once you have the answer to the difference between them, you will enter the when-to-use-each dilemma: When to use interfaces or abstract classes? When to use both?

I've limited knowledge of OOP, but seeing interfaces as an equivalent of an adjective in grammar has worked for me until now (correct me if this method is bogus!). For example, interface names are like attributes or capabilities you can give to a class, and a class can have many of them: ISerializable, ICountable, IList, ICacheable, IHappy, ...