React.js is a powerful and widely used JavaScript library for building user interfaces, particularly for single-page applications where dynamic and responsive user experiences are essential. Developed and maintained by Facebook, React has gained immense popularity within the web development community for its declarative and component-based approach. React follows a modular and reusable component-based architecture, allowing developers to break down complex UIs into smaller, self-contained components. Each component manages its state and behavior, making it easier to develop, test, and maintain code.