What is the difference between Storybook and React.js?

The difference between Storybook and React.js lies in their purpose and functionality. React.js is a JavaScript language library used to design the user interface, especially for developing reusable UI components. It’s a core library that renders UI components, the state of components, and web application interactions.
Storybook is one of those development tools that extend React’s functionality by attempting to provide a way for developers to create, test, and document their various UI components in isolation. This means setting up an environment outside of the main application, which might render components in its different visual states, with various props and styles.
React.js generally is the UI component building framework, while Storybook provides the development capability to enable a developer to develop, test, and document isolated UI components independently of the main application. Many developers use them together: React for the creation of UIs and Storybook for component management and documentation.