Bootstrap is a mobile-first framework consisting of HTML5, CSS3, and JavaScript code providing a comprehensive set of world class, responsive web design components. It was developed by engineers at Twitter and then released to the world in August 2011.
The framework includes code to retrofit modern features onto older browsers, a responsive 12-column grid system, and a long list of components (some using JavaScript) for building web applications and websites. It's meant to provide a strong foundation on which to build your application.
Refer to http://getbootstrap.com for more details.
