React is a JavaScript library for building user interfaces that uses a component-based approach. Components allow the user interface to be split into independent, reusable pieces. Data flows in one direction through the application via actions and a centralized state, rather than having two-way data binding. This unidirectional data flow model helps avoid bugs and makes the app more predictable and efficient. React uses a virtual DOM for rendering components, allowing it to efficiently update the real DOM when data changes.