What is BootStrap

Bootstrap is an open-source Javascript framework developed by the team at Twitter. It is a combination of HTML, CSS, and Javascript code designed to help build user interface components. Bootstrap was also programmed to support both HTML5 and CSS3.
Also it is called Front-end-framework.
Bootstrap is a free collection of tools for creating a websites and web applications.
It contains HTML and CSS-based design templates for typography, forms, buttons, navigation and other interface components, as well as optional JavaScript extensions.
Some Reasons for programmers preferred Bootstrap Framework

1. Easy to get started
2. Great grid system
3. Base styling for most HTML elements(Typography,Code,Tables,Forms,Buttons,Images,Icons)
4. Extensive list of components
5. Bundled Javascript plugins