Bootstrap is a web development framework that was first developed by Twitter. It is free and open-source, and is very popular when it comes to designing websites and web applications. It offers HTML and CSS based design templates for typography, forms, buttons, navigation, and other interface components. One different fact about bootstrap is that unlike many frameworks, it is concerned only with front-end development. Despite this, it is the most popular HTML, CSS, and JS framework for developing responsive, mobile first projects on the web.