The squarish USB plug and socket (port). Type B ports are found on USB hubs, printers, scanners and other peripheral devices. Cables plug into the peripheral using a Type B connector and into the computer with Type A.
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.
Eventually, a cable with Type C at both ends is expected to replace the A-B cable because Type C devices can detect their host-peripheral relationship. See USB Type C