React is a popular JavaScript library developed by Facebook for building user interfaces, particularly for single-page applications. It allows developers to create reusable UI components, making the development process more efficient and organized. With its virtual DOM feature, React optimizes rendering and improves performance, ensuring a smooth user experience. React's component-based architecture encourages the creation of modular code, which can be easily maintained and scaled. This has made it a favorite among developers for creating dynamic web applications.