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.”