Twitter Bootstrap is a front-end toolkit to rapidly build web applications. It is a framework that uses some of the latest browser techniques to provide you with stylish typography, navigation, buttons, tables, etc. One of the primary changes in Bootstrap 3 is an emphasis on Mobile-First responsive design. The goal is to elevate the mobile experience to a first-class citizen of the design process, because several billion mobile users is quite a large market to tap into. So, sites built with the current Bootstrap version target mobile devices and scale for larger screens depending on screen size.
This Bootstrap layout was made @Bootply. Bootply
The sizing "grid" is not the only aspect of responsive design. Making images and media object scale correctly is another consideration for responsive developers.
Go ahead and shrink your browser's width to see how the HTML elements and images respond as the width of the viewport changes.