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, commonly found in object-oriented programming, are essential for establishing a base structure that defines common interfaces for derived classes without allowing direct instantiation. They serve as blueprints, enabling developers to enforce a contract on derived classes, ensuring they implement specific methods. So while abstract classes can't be instantiated themselves, they pave the way for more specialized objects. In practical applications, abstract classes are often used in frameworks or libraries where you want users to extend your functionality. For example, in a graphics software application, you might create an abstract class called `Shape`, which has methods like `draw()` and `resize()`. Users can then create concrete classes like `Circle` and `Square`, which inherit from `Shape` and implement these methods, allowing for flexible and organized code that’s easier to maintain and extend.