Wikipedia says the following: “React (sometimes styled React.js or ReactJS) is an open-source JavaScript library providing a view for data rendered as HTML. React views are typically rendered using components that contain additional components specified as custom HTML tags. React promises programmers a model in which sub-components cannot directly affect enclosing components (“data flows down”); efficient updating of the HTML document when data changes; and a clean separation between components on a modern single-page application”.

React competes with Angular. Angular is more full-featured and has a larger following than React, but things are constantly changing.

React Native

You can build mobile apps with React Native using only JavaScript. React Native libraries were announced by Facebook in 2015, providing the React architecture to native iOS, Android and UWP applications.

React Native has a Getting Started page that guides you through the process of getting set up to build Android apps and iOS apps.