Facebook developed React, which is a Javascript front end library. React is a user interface tool that allows you to create web-based UI components. However, if this were all React did then it would be nothing more than a template library. React uses a syntax similar to HTML called JSX for developers to define components. These encapsulated components manage their own states, which makes it easy to pass rich information to a component and keeps the state of the application (and its components) out of the Document Object Model. React components are able to respond dynamically and effectively to changes to their state. React allows you to combine simple components into complex web applications.