React-Native is a framework that can be used to develop Android & iOS applications and whereas React.js is a parent JavaScript library can be used to develop web applications. In React-Native, the tags like <View>, <Text> are used very frequently and React.js, on the other hand, uses web HTML tags like <div> <h1> <h2>. There are only synonyms in the dictionary of web/mobile developments. For React.js, you need DOM for path rendering of HTML tags whereas for mobile application, React-Native uses AppRegistry to register your app.
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Also, check out this video for more similarities and differences between React.js and React-Native: