An ABC corresponds to an abstract concept. If you asked a Mechanic if he repaired Vehicles, he'd probably wonder what *kind* of Vehicle you had in mind. Chances are he doesn't repair space shuttles, ocean liners, bicycles, and volkswaggon beetles too. The problem is that the term 'Vehicle' is an abstract concept; you can't build one until you know what kind of vehicle to build. In C++, you'd make Vehicle be an ABC, with Bicycle, SpaceShuttle, etc, being subclasses (an OceanLiner is-a-kind-of-a Vehicle).
In real-world OOP, ABCs show up all over the place. Technically, an ABC is a class that has one or more pure virtual member functions (see next question). You cannot make an object (instance) of an ABC.
