The Bootstrap framework is a library of CSS and JavaScript for creating a website with a clean, uniform look. It includes many commonly used components — navigation bars, dropdown menus, alert messages. Bootstrap is "mobile-first," in that the developer creates a single site that displays well on any screen, from mobile devices to TVs. A grid system adjusts arrays of items (e.g., thumbnail images) to be wide and short on larger screens, but narrow and scrollable on smaller ones.

Bootstrap is a front-end framework (a complementary back-end framework being, e.g., Django). In using such frameworks, a developer is effectively outsourcing most of the technical work to the framework creators. Since the web is an ever-changing set of technologies and standards, it's nice to let the framework development communities chart the course.

As you might guess, the front-end uses Bootstrap!