Meta's UI library that defined two generations of frontend. Server Components and the React Compiler ship in React 19 / 20; powers Next.js, Remix, React Native.
From Wikipedia
React is a free and open-source front-end JavaScript library that aims to make building user interfaces based on components more "seamless". It is maintained by Meta and a community of individual developers and companies. According to the 2025 Stack Overflow Developer Survey, React is one of the most commonly used web technologies.