The squarish USB plug and socket. USB Type B sockets are found on USB hubs, printers, scanners and other peripheral devices. A-to-B cables plug into the computer (Type A) and peripheral (Type B).
Type A and B Prevent Accidental Hub Loops
The pins of Type A and B connectors carry the same signals, but their different plug-socket shapes prevent a loop from being created. For example, if a Type A connector were at each end of the cable, a cable from the computer to a hub and another cable back to the computer would cause a loop.
Cables with USB Type C at both ends are expected to replace the A-B cable because USB-C devices can detect their host-peripheral relationship. See USB Type C