From http://twitter.github.com/bootstrap/ : "Simple and flexible HTML, CSS, and Javascript for popular user interface components and interactions."

AKA: Allowing Programmers to take their website designs from nuclear-waste-bad to looks-decent-but-so-much-like-that-other-bootstraped-site-good

It's a design framework from Twitter for web apps. It has the many different components that you'll probably need (buttons, menus, icons, CSS grid, modals, progress indicators, alerts, etc). Similar to how GUI apps gets all those things "for free".