What is Bootstrap?
Bootstrap is an open-source Javascript framework developed by the team at Twitter. It is a combination of HTML, CSS, and Javascript code designed to help build user interface components. Bootstrap was also programmed to support both HTML5 and CSS3.
Also it is called Front-end-framework.
Bootstrap is a free collection of tools for creating a 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.
Some Reasons for programmers preferred Bootstrap Framework
Easy to get started
Great grid system
Base styling for most HTML elements(Typography,Code,Tables,Forms,Buttons,Images,Icons)
Extensive list of components
Bundled Javascript plugins
Taken from About Bootstrap Framework
It is an HTML, CSS, and JavaScript open-source framework (initially created by Twitter) that you can use as a basis for creating web sites or web applications.
More information and links to download
Getting started
Examples
Themes
Bootply - Bootstrap Editor and Builder
Update
The official bootstrap website is updated and includes a clear definition.
"Bootstrap is the most popular HTML, CSS, and JS framework for developing responsive, mobile first projects on the web."
"Designed and built with all the love in the world by @mdo and @fat."
Bootstrap, as I know it, is a well defined CSS. Although using Bootstrap you could also use JavaScript, jQuery etc. But the main difference is that, using Bootstrap you can just call the class name and then you get the output on the HTML form. for eg. coloring of buttons shaping of text, using layouts. For all this you do not have to write a CSS file rather you just have to use the correct class name for shaping your HTML form.