Bootstrap is a popular front-end framework for building responsive mobile-first websites and web applications. It provides pre-compiled and source code files that include HTML and CSS templates, as well as optional JavaScript plugins. The Bootstrap framework follows a mobile-first approach to make web content adaptive across a wide range of devices. It uses a 12-column responsive grid system for layout and includes many pre-built components and plugins for common tasks like navigation, typography, forms, buttons, images, alerts and more.