React.js is a JavaScript library for building user interfaces, used by Facebook and Instagram. It uses a virtual DOM for high performance. Node.js is a JavaScript runtime for building server-side applications and APIs. It allows JavaScript to execute on the server. Both are commonly used together, with React for the front-end and Node for the back-end. React is well-suited for applications with changing states, while Node is better for data-intensive server-side applications like streaming platforms.