Think of it as a partially complete class - you have to extend it to provide the functionality that it lacks before you can use it. Often such a class contains abstract methods that have to be implemented in the subclass, though sometimes a class is marked abstract just to prevent people instantiating it directly. Actor is one such example, it has no abstract methods that need to be implemented but it makes no sense to create a generic "Actor" - you always want a subclass of it. They're useful to avoid code duplication primarily - think of all the methods in Actor that you have for free whenever you create a subclass (move() for instance.) You don't ever create an actor directly but it contains functionality that's useful to each subclass of actor.