React is a popular open-source JavaScript library used for building user interfaces, particularly single-page applications (SPAs), where you need a fast, interactive, and dynamic user experience. Developed and maintained by Facebook, React focuses on creating reusable UI components that make the development process more efficient. It allows developers to build large-scale applications by breaking down the user interface into smaller, manageable parts, which can be reused across different parts of the app.

One of React’s standout features is its Virtual DOM. Instead of directly manipulating the real DOM (which can be slow), React creates a virtual representation of the DOM in memory and updates only the parts of the page that need to change. This makes React highly efficient in terms of performance, especially in applications with frequent updates. React also supports state management, allowing you to maintain and update application data seamlessly, and it works well with other libraries and frameworks, like Redux or React Router, to build more complex applications.