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 state management and rendering that state to the DOM, so creating React applications usually requires the use of additional libraries for routing. React Router is an example of such a library.
Definition from Wikipedia – React (web framework)