React is a JavaScript library for building user interfaces. It uses a component-based approach where UI is broken into independent, reusable pieces. The key principles of React include breaking UI into components, unidirectional data flow, identifying UI state, and dispatching actions to change state. React uses JSX syntax which resembles HTML but integrates JavaScript. Components can be "smart" or "dumb", with smart components providing data and dumb components displaying it. Redux is often used with React to manage state in a centralized store using actions and reducers. Debugging tools like React Developer Tools and Redux DevTools help develop React applications.