Abstract classes: Contain at most one pure virtual function. Cannot have abstract derived classes. Can have objects instantiated from them if the proper permissions are set. Are defined, but the programmer never intends to instantiate any objects from them.

Abstract classes are a fundamental part of object-oriented programming, allowing developers to define a blueprint for other classes. They can contain both abstract methods (pure virtual functions) and concrete methods. While they cannot be instantiated directly, we can create derived classes that implement the pure virtual functions, allowing us to create specific objects that leverage the shared interface and functionality. In real-world software development, abstract classes are invaluable for designing frameworks or libraries. For instance, if you're creating a graphics library, you might have an abstract class named `Shape` with methods like `draw()` and `resize()`. Specific shapes like `Circle` or `Rectangle` would derive from `Shape`, providing their implementations. This approach enhances code organization and promotes reuse while ensuring that all shapes adhere to a common contract.