Bootstrap is a free and open-source CSS framework directed at responsive, mobile-first front-end web development. It contains CSS- and (optionally) JavaScript-based design templates for typography, forms, buttons, navigation, and other interface components. Bootstrap's design templates are based on the principles of responsive design, allowing web developers to create websites and web applications that automatically adjust their layout and appearance based on the user's device and screen size. Bootstrap's components and utilities make it easy to build modern, visually appealing user interfaces without the need for extensive custom CSS or JavaScript code. Bootstrap is widely used by web developers and designers to streamline the process of building responsive, cross-browser compatible websites and web applications.