Bootstrap is an open source framework for designing responsive websites. Developed by Twitter in 2011, it is popular worldwide for faster web development. Yes, Bootstrap is a front-end framework. While using Bootstrap, you work with HTML, JavaScript and CSS. Mobile-first styles are PART of the core framework.

Bootstrap is called a framework since it is the conceptual structure intended to serve as a support or guide for DESIGN of responsive websites that expands the structure into something useful. For example, a real-life example is the structure of a BUILDING while constructing it.

There has been a lot of debate on whether Bootstrap is a framework or LIBRARY, but we can consider it as a front-end library. While adding CSS, we include bootstrap.css or bootstrap.min.css. Including a file of the same FORMAT itself says that you are adding a framework. These are prewritten and addition of these files makes a new framework. Therefore, Bootstrap is a framework and library in its correct sense.