React is a JavaScript library for building user interfaces using reusable components. It is used to create single page applications that dynamically update the current page with new data from the server. React uses a component-based approach and one-way data binding to build interfaces simply and allow for easy testing. Key concepts in React include components, props, state, lifecycles, hooks, JSX, and the virtual DOM. Major companies using React include Facebook, Netflix, Instagram, and WhatsApp.