Twitter Bootstrap is a freely available front-end framework for developing responsive, mobile-first websites and web applications. It contains HTML and CSS templates for common user interface components like buttons, navigation, and grids, as well as optional JavaScript extensions. Developers use Bootstrap because it speeds up development by providing pre-built templates and components, has a large user community for support, and is optimized for cross-browser compatibility including older browsers. The framework can be easily customized and integrated with other libraries and tools.