React is an open source JavaScript library for building user interfaces. It was created by Jordan Walke at Facebook in 2011 and is now maintained by Facebook, Instagram, and a community of developers. Major companies like Facebook, Netflix, Instagram, Khan Academy, and PayPal use React to build their interfaces. React uses a virtual DOM for faster rendering and makes components that manage their own state. It uses JSX syntax and a one-way data flow that is declarative and composable.