React.js:
React.js often referred to as React or ReactJS is a JavaScript library responsible for building a hierarchy of UI components or in other words, responsible for the rendering of UI components. It provides support for both frontend and server-side.
You can reuse code components in React JS, saving you a lot of time.The rendering of your pages completely, from the server to the browser will improve the SEO of your web app.
React Native:
React Native is a framework for building native applications using JavaScript. React Native compiles to native app components, which makes it possible for you to build native mobile applications. In React JS, React is the base abstraction of React DOM for the web platform, while with React Native, React is still the base abstraction but of React Native. So the syntax and workflow remain similar, but the components are different. React Native comes with all the advantages that React.js brought you. React.js focuses on a better UI, so those benefits remain.