Bootstrap is a front-end framework for developing responsive and mobile-first web pages. Originally developed by Twitter, Bootstrap provides a collection of CSS and JavaScript components, such as navigation bars, buttons, forms, modals, and grids, that can be easily integrated into web projects. Bootstrap's responsive grid system allows developers to create flexible and adaptive layouts that work well on various screen sizes and devices. It also includes utility classes and themes to quickly style web pages consistently. Bootstrap simplifies the development process, enhances the appearance of web pages, and ensures a consistent user experience across different devices.