Bootstrap is a responsive front-end framework which use for development of responsive websites.
It mainly design to work on responsive manner means bootstrap automatically scales according to device.
One code base can easily and efficiently work on desktops, mobile, Pad etc. It contains HTML and CSS templates like forms, buttons, navigation , grid etc.
Millions of websites using Bootstrap due to its flexibility, scalability , continue enhancement, community support etc.
Bootstrap also support pre-processors like Less and Sass.