Storybook is a tool used to develop and design user interfaces (UI) in JavaScript. It allows developers to create components that can be reused throughout their application, as well as visualize how the UI will look and behave before it goes live. Storybook also enables developers to quickly prototype and iterate on their designs with hot-reloading of changes. Storybook also offers a number of add-ons that allow developers to easily integrate testing and automation tools into their workflow.