Bootstrap is a front-end framework that simplifies the process of designing responsive and mobile-first websites. It includes pre-designed components like buttons, modals, and navigation bars, as well as a grid system for layout design. Bootstrap supports customization through its SCSS variables and mixins, making it adaptable to various projects. Its compatibility with all major browsers and ease of use have made it one of the most popular frameworks for web development. Developers use Bootstrap to quickly prototype and build clean, consistent designs.