USB-B is the thing that's on the other side of cables with USB-A. A large, square-ish port that got even larger when it was extended for USB 3, with a connector that got even larger. That's the port that mini-USB and micro-USB are mini and micro versions of.

You can find USB-B ports on e.g. laser printers (where size doesn't matter) or on some older USB drives (where you'd want USB 3 speeds).