React is a popular JavaScript library used for building user interfaces. It was developed by Facebook and is now maintained by a community of developers. React allows developers to create reusable UI components and efficiently update the user interface in response to changes in data or user interactions. It uses a declarative approach to programming, where developers describe what they want the UI to look like and React takes care of updating the DOM. React is often used in conjunction with other libraries and frameworks, such as Redux and React Native, to build complex web applications and mobile apps.