React is an open source JavaScript library that provides a view-layer for rendering data as HTML to create interactive UI components. Components have been used typically to render React views that contain additional components specified as custom HTML tags. React views efficiently update and re-render the components without reloading the page when your data changes. It gives you a trivial virtual DOM, powerful views without templates, unidirectional data flow, and explicit mutation. It is a very systematic way of updating the HTML document when the data changes and provides a clean separation of components in a modern, single-page application.

The React Component is built entirely with Javascript, so it's easy to pass rich data through your app. Creating components in React lets you split the UI into reusable and independent pieces, which makes your application component reusable, testable, and makes the separation of concerns easy.

React is only focused on View in MVC, but it also has stateful components that remember everything within this.state. It handles mapping from input to state changes and it renders components. Let's look at React's component life cycle and its different levels.