Muddiest Point: Lecture 2

Most of Lecture 2 was straightforward, so there weren't too many muddy points to choose from. Out of curiosity, though, I have to ask what the connection is between USB cables and computer buses. The lecture made it seem as though buses are purely channels that allow devices within a computer to communicate with each other. If that's the case, then what makes cables that connect devices to computers externally "Universal Serial Buses"?