What is Bootstrap (front-end framework) ?

It is a free collection of tools for creating websites and web applications. Bootstrap contains HTML and CSS-based design templates for typography, forms, buttons, navigation and other interface components, as well as optional JavaScript extensions. In March 2014 it was the No.1 project on GitHub with 65,000+ stars and 23,800 forks, with a user base including MSNBC and NASA.
Bootstrap was developed by Mark Otto and Jacob Thornton at Twitter as a framework to encourage consistency across internal tools. Before Bootstrap, various libraries were used for interface development, which led to inconsistencies and a high maintenance burden.
According to Twitter developer Mark Otto: super small group of developers and I got together to design and build a new internal tool and saw an opportunity to do something more. Through that process, we saw ourselves build something much more substantial than another internal tool. Months later, we ended up with an early version of Bootstrap as a way to document and share common design patterns and assets within the company.”
The first deployment under real conditions happened during Twitter’s first Hackweek. Mark Otto showed some colleagues how to accelerate their project’s development with the help of the toolkit. As a result, dozens of teams have moved to the framework.

In August 2011, Twitter released Bootstrap as open source. In February 2012, it was the most popular GitHub development project.
Bootstrap is compatible with the latest versions of all major browsers. It gracefully degrades when used on older browsers such as Internet Explorer 8.
Since version 2.0 it also supports responsive web design. This means the layout of web pages adjusts dynamically, taking into account the characteristics of the device used (desktop, tablet, mobile phone).
Starting with version 3.0, Bootstrap adopted a mobile first design philosophy, emphasizing responsive design by default.
Bootstrap is open source and available on GitHub. Developers are encouraged to participate in the project and make their own contributions to the platform.
Recently, community members have translated Bootstrap’s documentation into various languages, including Chinese, Spanish and Russian.