Storybook

Storybook is a user interface development tool for React.js which allows developers to build, display, and test varying states of UI components in an isolated and organized environment. It provides a simple way to view different states of your React components and develops complex UIs from simple building blocks.

At its core, Storybook lets you develop and view your components outside of your application. This brings several benefits: you don't need to worry about app-specific dependencies, and developing in isolation allows you to focus solely on the component and nothing else.

Let's take an example of a Button component snippet with storybook:

```jsx
import React from 'react';
import { Button } from './Button';

export default {
title: 'Button',
component: Button
};

export const Text = () => <Button onClick={() => {}}>Hello Button</Button>;

export const Emoji = () => (
<Button onClick={() => {}}>
<span role="img" aria-label="so cool">
😀 😎 👍 💯
</span>
</Button>
);
```

In the code above, the default export defines meta information about the Button component (like its title), and the other exports define the different states of the component. The Text component is a state of Button containing text, and the Emoji component is another state of Button containing emojis.

These different states — defined as different React functions — are referred to as "stories."

To use Storybook in a React Application, you'd usually install it via NPM, initialize it, create a .storybook folder in your project root, and run the storybook command to start it.

Storybook is an essential tool for UI development and component-driven design, as it makes it easier to develop and test components independently.