React.js was developed by Facebook to address its need for a dynamic and high performing User Interface(UI). It was released in 2011 with an intent to leverage JavaScript speed and the ability to render pages and dynamic user input.
Two years later, as React.js became quite functional, the team open-sourced React. In 2015 they released React Native.
React Native was released in 2015 and became a really useful framework for building native applications using JavaScript since it can be used for Android and allows building cross-platform mobile apps. A few contributors also customised the framework to build desktop apps for Mac and Windows.
React Native compiles native app components, which enables building native mobile applications. So the syntax and workflow in React.js and React Native remain similar, but the components are different.