React.js is an open-source JavaScript library for building user interfaces or UI components. It is maintained by Facebook and a community of individual developers and companies. React.js uses a component-based architecture, allowing developers to break down complex user interfaces into reusable and composable components. It uses a virtual DOM (Document Object Model) to efficiently update the UI in response to changes in data, resulting in fast and responsive user experiences. React.js provides features such as JSX (JavaScript XML) syntax for writing declarative UI components, props for passing data between components, and state for managing component-specific data. It integrates seamlessly with other JavaScript libraries and frameworks, enabling developers to leverage existing code and tools. React.js is used by millions of developers worldwide to build interactive and dynamic web applications, from simple websites to complex single-page applications.