Class components are a fundamental part of the React library, used for building user interfaces. They are JavaScript classes that extend the base Component class from React. Class components can manage their own state and lifecycle methods, allowing developers to create dynamic and interactive applications. In a class component, you define a render method that returns the JSX to be displayed. Class components can also handle events and update their state using the setState method. This makes them suitable for more complex applications where managing state and lifecycle events is essential for functionality.