Bootstrap is an open source toolkit for developing responsive, mobile-first web sites and applications. It contains HTML and CSS-based design templates for components like forms, buttons, navigation, and other common user interface elements. Bootstrap was developed at Twitter to provide consistency across internal tools and was released publicly in 2011. It has a mobile-first approach, supports all major browsers, enables responsive designs, and is easy for developers to get started with using HTML and CSS. Bootstrap includes scaffolding, base CSS styles, reusable components, JavaScript plugins, and customizable options.