Twitter Bootstrap is a free and open-source front-end web framework that contains HTML and CSS-based design templates and components for typography, forms, buttons, navigation, and other interface components, as well as optional JavaScript extensions. It aims to provide a simple and flexible framework for building responsive, mobile-first projects. The document outlines the key components of Bootstrap including scaffolding, base CSS, JavaScript plugins, and provides examples of grids, buttons, dropdowns, and other elements.