React is a parent JavaScript library for building applications using JavaScript. React Native is an entire platform that enables you to develop native, cross-platform mobile applications, and React.js is a JavaScript library that allows you to use for building a high-performing UI layer.
In simple words, React is better for developing the dynamic, high-performing, responsive user interface for your web interfaces and whereas React Native is better to give your mobile apps a truly native feel.
If you are interested to learn React Js, check out this React Course by Intellipaat.
Also, check out this video for more similarities and differences between React.js and React-Native: