Bootstrap is a CSS framework that makes it easier to create website and web application user interfaces. Bootstrap is especially useful as a base layer of CSS to build sites with responsive web design.
What is Bootstrap and how do I use it? is an awesome tutorial that goes through many of the main Bootstrap elements such as icons, navigation bars and "jumbotron"-style webpages.
Bootstrap 4: What's New? shows a slew of comparisons for how user interface elements have changed design over the past several Bootstrap major version iterations.