From Wiki
Bootstrap is a free collection of tools for creating websites and web applications. It contains HTML and CSS-based design templates for typography, forms, buttons, navigation and other interface components, as well as optional JavaScript extensions.
It is the most popular project on GitHub and has been used by NASA and MSNBC among others.

Bootstrap was developed by Mark Otto and Jacob Thornton at Twitter as a framework to encourage consistency across internal tools. Before Bootstrap, various libraries were used for interface development, which led to inconsistencies and a high maintenance burden.

Bootstrap website

Bootstrap is simple to use. Look at few examples .

Bootstrap come with many ready to use features. See why to use at this link.