Bootstrap is a free framework for faster Web development. It was developed at Twitter and released as an open source framework in 2011. It is easy to use and uses responsive design so that Web pages look great on all devices. It is compatible with all modern browsers.
Bootstrap can be downloaded from getbootstrap.com or it can included from a content delivery network. Using a content delivery network (CDN), pages often load faster because users have likely already loaded a page that linked to the same resource.
Check out an excellent tutorial from https://www.w3schools.com/bootstrap/.
Complete documentation is available at http://bootstrapdocs.com/v3.0.0/docs/getting-started/.