Twitter Bootstrap for UI is to develop sleek, intuitive, and powerful front-end framework for faster and easier web development. Bootstrap works in PC, tablet and smartphone browsers via responsive CSS. Bootstrap for UI developed on top of HTML5/CSS3 and jQuery.

Main building blocks available with bootstrap is Components, Javascript extensions and Scaffolding.

Components

Bootstrap supports dozens of built-in responsive components. Few of the built-in components available are
• Dropdowns
• Button groups
• Button dropdowns
• Navs
• Navbar
• Breadcrumbs
• Pagination
• Labels & badges
• Typography
• Thumbnails
• Alerts
• Progress bars
• Media Objects

JavaScript

Bootstrap added lot of jQuery plugins and currently available 13 custom jQuery plugins listed down
• Transitions
• Modal
• Dropdown
• Scrollspy
• Tab
• ToolTip
• Popover
• Alert
• Button
• Collapse
• Carousel
• Typeahead
• Affix

Scaffolding

Bootstrap is built on responsive 12-column grids, layouts, and components
• Grid System
• Fluid grid System
• Layouts

BootSnipp

Bootsnipp defines the design elements and code snippets for Bootstrap HTML/CSS/JS framework. Community developed code snippets help the developers to design intuitive web pages faster and reuse the great designs.
http://bootsnipp.com/

Button builder (http://bootsnipp.com/buttons) in bootsnipp helps designing great bootstrap buttons with different size, color, icons and so on.

Download

Bootstrap can be downloaded from http://getbootstrap.com/2.3.2/
It can be downloaded from GitHub https://github.com/twbs/bootstrap

We can customize the bootstrap download by specifying required components, styles, font specifications and so on at http://getbootstrap.com/2.3.2/customize.html