Bootstrap is included in Joomla 3 by default - Bootistrap is an open-source framework which was initially developed by Twitter and is a combination of HTML, CSS, and Javascript files. Bootstrap supports both HTML5 /CSS3 and is now its third version as Bootstrap 3.

With Bootstrap it enables us to take advantage ( in standardisation, speed and functionality) of pre developed/tested CSS and javascript in developing responsive websites .

Files include (these have been adapted when being used in Joomla 3)

bootstrap.css
bootstrap.min.css
bootstrap-responsive.css
bootstrap-responsive.min.css
glyphicons-halflings.png
glyphicons-halflings-white.png
bootstrap.js
bootstrap.min.js

You can find much more about Boostrap by checking out getbootstrap.com/ or siteground www.sitepoint.com/11-reasons-to-use-twitter-bootstrap/