Object-oriented programming (OOP) is a programming paradigm that allows structuring programs as a collection of objects interacting with each other. OOP is based on four fundamental principles: encapsulation, inheritance, polymorphism, and abstraction.

1. Encapsulation:
Encapsulation allows hiding the implementation details of an object and providing only the necessary interface for interacting with it. This is achieved by combining data and methods that operate on this data inside the object. Thus, the object can control access to its data and methods, ensuring security and simplifying usage.

2. Inheritance:
Inheritance allows creating new classes based on existing classes, inheriting their properties and methods. This enables code reuse, reduces duplication, and establishes a class hierarchy. The parent class is called a superclass, and the child class is called a subclass.

3. Polymorphism:
Polymorphism allows objects of different types to be treated in a similar manner. This is achieved by allowing objects of different classes to have the same methods with different implementations. Polymorphism simplifies the code and makes it more flexible.

4. Abstraction:
Abstraction allows hiding unnecessary implementation details and focusing on the key aspects of an object. This helps create more understandable and easily maintainable code. Abstraction also enables the creation of abstract classes and interfaces that define common characteristics for subclasses.

Applying the principles of OOP enables the creation of modular, flexible, and scalable programs. OOP is widely used in software development as it simplifies the development process, promotes code reuse, and enhances its structure.