Another older introduction can be seen here: React: Rethinking best practices.
7 Minute intro to coding with it: https://egghead.io/lessons/react-react-in-7-minutes
React (web framework)
React can be used as a base in the development of single-page or mobile applications. However, React is only concerned with rendering data to the DOM, and so creating React applications usually requires the use of additional libraries for state management and routing. Redux and React Router are respective examples of such libraries.
Definition from Wikipedia – React (web framework)