Freek Lijten: SOLID - The D is for Dependency Inversion Principle
The DIP deals with avoiding mixing different levels of abstraction in your code. In this article we will explore the last of these principles in depth. [...] I want to show you how depending on abstractions makes our lives easier in a larger example. Since we're going to be loading, updating and storing bikes a lot in our application I would like an Api for that. It would not be such a good idea to write the same queries or perform the same validation over and over.
He includes two examples - one a bit simplistic involving bike storage and another more complex that uses interfaces to define structure and object injection rather than passing data around. He also tries to help clear up some of the confusion that might happen around dependency inversion and dependency injection.