This week's book giveaway is in the General Computing forum. We're giving away four copies of Arduino in Action and have Martin Evans, Joshua Noble, and Jordan Hochenbaum on-line! See this thread for details.
Felix, You use an interface when you are defining a contract for the class. For example, "all Display objects will implement print()."
You use an abstract class when you want to provide some base functionality. You also use an abstract class when you want to provide a template and the caller just has to fill in a few methods that get called in the middle.
I use abstract classes that don't contain a single non-abstract method as well because some classes will always be only one thing and are never going to be another at the same time. I use abstract classes in this case in order to minimize the sources of bugs and bad design (classes that do too much).