There is a plethora of amazing components that are provided with Bootstrap, thus saving time creating them from scratch. There are components for dropdowns, buttons, images, and so much more.
The usage is very similar to that of the grid system, and the same HTML elements we know and love are used with CSS classes to modify and display Bootstrap constructs. I won't go over every component that Bootstrap offers as that would require an encyclopedia in itself, and many of the commonly used ones will be covered in future chapters through example projects. I would however recommend taking a look at some of the components on Bootstrap's website https://v4-alpha.getbootstrap.com/components/alerts/