Bootstrap, a css framework that developed by Twitter, created by Mark Otto and Jacob Thronton. Which original was the internal framework in twitter for their products and quick prototyping.
On the github page, Bootstrap is described as:
That is, Bootstrap is not only just a css framework, but actually an UI framework base
UI elements like dropdown, popover and tooltips. Evenmore, all of the color and elements is customizable because the css is generated by less, all setting is in varibles.less file and allow user to change the variables or reuse the elements class in less.