ReactJS is an open-source JavaScript library used for building user interfaces, particularly for single-page applications. Developed by Facebook, React allows developers to create reusable UI components that efficiently update and render when data changes. It uses a virtual DOM (Document Object Model) to optimize rendering performance, making React fast and responsive. React's component-based architecture and declarative syntax simplify the development of interactive and dynamic web applications, making it popular for building modern, scalable, and maintainable web apps.