What is Bootstrap?
Bootstrap is a platform for building mobile-friendly ("mobile-first") websites and includes HTML, CSS, and jQuery libraries. It delivers websites built with responsive design which means the elements on the page can re-arrange and/or resize based on the width of the screen.
For example, on a wide desktop screen a visitor might see a sidebar in the left or right column. Then, on a mobile device that sidebar might either disappear or move to the bottom of the screen, below the content. Responsive design provides for a much improved experience for all visitors - especially for those on mobile devices.
Recently, Google began ranking websites higher in search results if the site is mobile-friendly. This, along with the improved visitor experience, are strong indications that all websites should be mobile-friendly.
While Bootstrap does not include a back-end administrative feature like CMS sites, most current CMS sites use templates or themes that are based in Bootstrap.