We built React to solve one problem: building large applications with data that changes over time.
Simply express how your app should look at any given point in time, and React will automatically manage all UI updates when your underlying data changes.
When the data changes, React conceptually hits the "refresh" button, and knows to only update the changed parts.
React is all about building reusable components. In fact, with React the only thing you do is build components. Since they're so encapsulated, components make code reuse, testing, and separation of concerns easy.
React challenges a lot of conventional wisdom, and at first glance some of the ideas may seem crazy. Give it five minutes while reading this guide; those crazy ideas have worked for building thousands of components both inside and outside of Facebook and Instagram.
You can learn more about our motivations behind building React in this blog post.