Bootstrap is the most popular HTML, CSS, and JavaScript framework for building responsive, mobile-first web applications, developed originally by Twitter and maintained as an open-source project. It provides a comprehensive 12-column grid system, extensive pre-built components, and powerful utility classes that accelerate development while ensuring consistent, professional design across all devices. Key to effective Bootstrap usage is understanding that utility classes trump custom CSS in most scenarios — combining them strategically creates flexible, maintainable layouts without writing verbose stylesheets, while the framework's breakpoint-driven responsive approach ensures seamless adaptation from mobile phones to ultra-wide displays.