Bootstrap has been ruling the CSS world for quite a long time. It was introduced in the year 2011 as a front end framework which would ease the design pain which most developers experience when designing a new website. As to what it is
Build responsive, mobile-first projects on the web with the world’s most popular front-end component library.
Bootstrap is an open source toolkit for developing with HTML, CSS, and JS. Quickly prototype your ideas or build your entire app with our Sass variables and mixins, responsive grid system, extensive prebuilt components, and powerful plugins built on jQuery.
That’s the site definition of what Boostrap is. However to me, its set of CSS style using which I can quickly get a nice looking web system or a website up and running.