Components are the essential building blocks of all our web pages. The grid system deals with the underlying structure; the Javascript modules add some interactivity and functionality, but the components are the essential elements of the pattern library.
They're (roughly) organised into atoms, molecules and organisms. In general, an atom is a single piece of code, a combination of two or three elements, bound together in a coherent package, like a button. A molecule is a more complicated combination of atoms, like pagination, and an organism is the most complex of all, perhaps combining several molecules together, such as a form.