Object-oriented programming (OOP) is a programming paradigm that is focused on using objects to represent and manipulate data. It organizes code into objects, which can have properties and methods.

Here are some key concepts in object-oriented programming:

1. Classes: Classes are blueprints or templates for creating objects. They define the properties (attributes) and behaviors (methods) that objects of that class will have.

2. Objects: Objects are instances of a class. They represent real or conceptual entities and contain data (state) and behavior. Objects can interact with each other through messages.

3. Encapsulation: Encapsulation is the concept of bundling data and the code that operates on that data together into a single entity (an object). It helps in hiding the internal details of an object and provides a clear interface for interacting with it.

4. Inheritance: Inheritance allows objects to inherit properties and behaviors from a parent class. It promotes code reusability and enables the creation of more specialized classes (child classes) based on existing ones (parent classes).

5. Polymorphism: Polymorphism means that objects of different classes can be treated as objects of the same parent class. This allows for code flexibility and allows objects to respond differently to the same message based on their specific implementation of the method.

6. Abstraction: Abstraction involves simplifying complex systems by breaking them down into their essential parts. In OOP, abstraction is achieved through abstract classes and interfaces, which provide a high-level view of an object’s functionality without detailing its implementation.

7. Message Passing: Objects communicate with each other by sending messages. This involves calling methods on objects to perform specific actions or retrieve information. The receiver object then responds by executing the appropriate method.

These concepts are the building blocks of object-oriented programming and are used to create modular, reusable, and maintainable code. Understanding these concepts is crucial for effectively designing and implementing object-oriented systems.