You could say that React.js is the V in MVC, i.e. the part that is meant to render your views.
Differences between React.js and React Native
In React JS, React is the base abstraction of React DOM for the web platform, whilst in React Native, React is the base abstraction of React Native. So syntax and workflow are similar but components are different.
React.js is the foundation of React Native and uses the same principles and syntax, so it should be fairly easy to learn React Native if you already know React.js.