A group of closely related classes is called a hierarchy or a class hierarchy.

In object-oriented programming, classes are used to define objects and their behavior. When multiple classes share common characteristics and behaviors, they can be grouped together into a hierarchy. This allows for better organization, code reusability, and abstraction.

Benefits of using a class hierarchy:
1. Code Reusability: By grouping related classes together, we can reuse code across different classes. The common behaviors and attributes can be defined in the superclass, and the subclasses can inherit those properties. This reduces code duplication and makes the codebase more maintainable.

2. Abstraction: A class hierarchy allows us to define abstract classes or interfaces at the higher level of the hierarchy. These abstract classes can provide common behavior and attributes, but they cannot be instantiated directly. Subclasses must implement the abstract methods defined in the abstract class. This provides a level of abstraction and helps in designing flexible and modular code.

3. Polymorphism: In a class hierarchy, subclasses can override the methods defined in the superclass. This allows for polymorphism, where objects of different classes can be treated interchangeably. The superclass can define a method, and each subclass can provide its own implementation. This increases flexibility and allows for dynamic behavior based on the actual type of the object.

Example:
Let's consider a class hierarchy for animals. We can have a superclass called "Animal" and subclasses like "Mammal," "Bird," and "Fish." Each subclass can inherit the common attributes and behaviors defined in the "Animal" class, such as "name," "age," and "eat()." The subclasses can also have their own specific attributes and behaviors. For example, the "Mammal" class can have a "furColor" attribute and a "milk()" method.

Summary:
A group of closely related classes is called a hierarchy or a class hierarchy. It allows for code reusability, abstraction, and polymorphism. By organizing classes into a hierarchy, we can create a modular and maintainable codebase. In the example of an animal hierarchy, the superclass "Animal" can define common attributes and methods, while subclasses like "Mammal," "Bird," and "Fish" can inherit and extend those properties.