React is an open-source, front end, JavaScript library for building user interfaces or UI components. It is maintained by Facebook and a community of individual developers and companies. React can be used as a base in the development of single-page or mobile.

Sites build with react:

1. Facebook
2. Instagram
3. Netfilx
4. Uber
5. Reddit

How to learn react
PDF ebook Learning React JS
https://oiipdf.com/learning-react