A Key Element in Responsive Websites
Articles about responsive design use terms like Twitter Bootstrap so for anyone who may be wondering, Bootstrap is the most popular HTML, CSS, and JS framework for developing responsive, mobile first projects on the web. The following from http://getbootstrap.com provides an overview:
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.
One framework, every device.
Bootstrap easily and efficiently scales your websites and applications with a single code base, from phones to tablets to desktops with CSS media queries.
Full of features
Bootstrap is open source. It's hosted, developed, and maintained on GitHub.