Storybook is an open-source tool used for developing and testing user interface components in isolation. It allows developers to create and showcase UI components without the need for a full application, making it easier to visualize and test different states and variations of components. With Storybook, developers can build components for various frameworks like React, Vue, and Angular. It provides a user-friendly interface where designers and developers can collaborate, ensuring that components are consistent and functional before they are integrated into larger projects.