ReactJS is a JavaScript library, supporting both web interface and running on a server, for building user interfaces and web applications. Follow the concept of reusable components.

React Native is a mobile framework that uses the JavaScript engine available on the host, allowing you to build mobile apps for different platforms (iOS, Android, and Windows Mobile) in JavaScript that allows you to use ReactJS to create reusable components and communicate. with native components additional explanation

They both follow the JSX syntax extension of JavaScript. Which compiles React.createElement calls under the hood. JSX in depth

Both are open sourced by Facebook.