It is positioned as a “UI library” to help developers construct user interfaces through components. React also provides the ability to store an application state and update UI accordingly.
It was developed internally at Facebook with contributions from the open source community. Dan Abramov is one of those developers at Facebook.
He’s taken the time to write a very thorough developer centric post on how React works. I know only enough React to build something simple, but this really helped me understand what is happening under the hood.