Bootstrap is the world's most popular open-source CSS framework for building responsive, mobile-first websites and web applications. Developed by Twitter engineers in 2011, it provides a comprehensive collection of pre-built components, utilities, and a flexible grid system. Bootstrap includes styled components for navigation bars, buttons, forms, cards, modals, alerts, and dozens of other interface elements with consistent design patterns. The framework uses a 12-column responsive grid system that adapts layouts to different screen sizes using breakpoints. Bootstrap is built with Sass preprocessor, allowing developers to customize variables, colors, and component styles. Version 5 removed jQuery dependency, making it lighter and more modern. The framework is widely used in rapid prototyping, admin dashboards, corporate websites, and as a foundation for custom design systems.