ReactJSReactJS is a javascript libraryjavascript library which is used to build web interfaces. You would need a bundler like webpack and try to install modules you would need to build your website.
React NativeReact Native is a frameworkjavascript framework and it comes with everything you need to write ios and androidmulti-platform apps built in(like iOS or Android). You would need xcode and android studio installed to build and rundeploy your app.
Unlike ReactJS, React-Native doesntdoesn't use HTML but similar components that you can use accross ios and android to build your app. These components use real native components to build the ios and android apps. Due to this React-Native apps feel real unlike other Hybrid development platforms. Components also increases reusability of your code as you dontdon't need to create same user interface again on ios and android.
I would recommend all ios and android developers to have fun with this library.