Bootstrap is a front-end framework, a library of ready-made CSS and JavaScript that gives a website its layout, typography and interface parts, so that a team does not write them from zero. Mark Otto and Jacob Thornton created it, and the Bootstrap team and contributors maintain it.
A page includes Bootstrap's CSS file and JavaScript bundle, for example from a CDN (a content delivery network that serves the files from servers near the visitor), and then uses its class names in the HTML. The framework provides a responsive grid that rearranges columns for phone and desktop screens, components such as navigation bars, buttons, cards, forms, modals and carousels, and JavaScript plugins for dropdowns, popovers and tooltips. Designers change colors and sizes through Sass, a language that compiles to CSS. Version 5.3 is the current stable line; version 6 is in alpha.
Bootstrap is free and open source. The code is released under the MIT License and the documentation under Creative Commons.