Bootstrap is an intuitive and very powerful front-end framework. Twitter Bootstrap is a free collection of tools for faster and easier web development. It was designed for developers of all skill levels.
As the name implies Bootstrap was originaly developed by Twitter for internal purposes. Today it is one of the most popular projects on GitHub.
Bootstrap has a lot of useful resources and features for faster and more intuitive web development:
Responsive 12-column grid
CSS styles and components
Lots of modular jQuery plugins
Responsive features according to resolutions and devices
Built on LESS
Complete styleguide documentation
There are two main versions out there: 3 RC1 & 2.3.2.
Version 3 is mobile first and drops responsive support for IE8. It also has completely different syntax so it's incompatible with version 2 projects.