What is it all about?
Bootstrap is open source. It's hosted, developed, and maintained on GitHub. It is the most popular HTML, CSS, and JS framework for developing responsive, mobile first projects on the web.
Bootstrap ships with vanilla CSS, but its source code utilizes the two most popular CSS preprocessors, Less and Sass. Quickly get started with precompiled CSS or build on the source. Bootstrap easily and efficiently scales your websites and applications with a single code base, from phones to tablets to desktops with CSS media queries. With Bootstrap, you get extensive and beautiful documentation for common HTML elements, dozens of custom HTML and CSS components, and awesome jQuery plugins.