Twitter 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.
The difference between Bootstrap and standard CSS designing, and also the reason of bootstrap's popularity, is that the person designing the page does not have to write CSS rules. He uses a set of predefined classes and styles and implements it and combines it.
The easiest way to learn it is to start form some examples provided at http://getbootstrap.com/ and combine it with other componenets provided in the documentation