Recs.
Updated
A toolkit providing simple and flexible HTML, CSS, and JS for popular UI components and interactions.
Specs
Pros
Pro Custom builds
If you don't want to download the full Bootstrap framework. Custom builds of Bootstrap can be created, including only the desired CSS, CSS components, and JavaScript components. This can be done directly from the Bootstrap website by simply choosing what components to download.
Pro Well documented
Documentation is thorough, well organized and full of live examples and templates ready for use. Every component and every part of the framework is explained and covered in depth.
Pro A wide variety of themes available
To help you avoid the "Bootstrap look", there are many resources that provide a great selection of themes and templates for Bootstrap.
Cons
Con Too many classes
Bootstrap's over-reliance on HTML classes for styling can get very messy very quickly. There's also an overabundance of DOM elements which have a lot of classes and are more often than not nested inside DOM elements with even more classes. This gets problematic down the line because the maintainability of the project gets harder when the project starts to get large.
Con Websites can start to look the same
If the initial Bootstrap colors and styles are not changed or edited, different websites start looking the same even if they have nothing to do with each other and they are made by different developers.
Con Hard to maintain
Bootstrap - both 1 and 2 - uses very common class names, like "alert", "label", "btn". Those short names are great when you're hand typing out your CSS, but they are very difficult to search for in a codebase of thousands of files. Also these classes may cause collision with other frameworks or components CSS.
Con Docs aren't in-depth enough
The docs include too few examples and too little details for each topic discussed. For example, it's unlikely you will be able to understand how modals work from five simplest possible use cases. What roles each of the wrappers play in the modal's HTML? What can be omitted and what must stay? What actually "hide" method does, does it remove the modal's data from DOM? And that applies to each and any of the widgets in Javascript section.
Con Rather heavy
Bootstrap is quite large compared to most CSS frameworks and if not minified, websites that use it suffer performance-wise.