Abstractions: The good and the bad

The concept of abstraction is a bit… abstract. What is it actually? A concept of object-oriented programming? A Java keyword? An artistic style? Or a phenomenon that surrounds us in everyday life and everyday code – something we could not create any software without?
 You too create abstractions every day, often without realizing it. But not all abstractions are created equal. What makes a good abstraction, and what makes a bad one? Let's explore together and widen our understanding of this core concept.