USB-A vs. USB-B

USB-A (Standard-A) is the flat rectangular connector on host ports -- computers, chargers, hubs. It used 4 pins for USB 1.x/2.0 speeds; a 9-pin variant, adding 5 SuperSpeed contacts, extended it to USB 3.0+ in 2008 without changing the external shape or requiring a new socket design.

USB-B (Standard-B) is the squarish connector historically used on printers, scanners, and external hard drives -- the peripheral-side counterpart to USB-A. A 9-pin SuperSpeed variant with a distinctive stacked upper tab was added for USB 3.0, though USB-C has since displaced Standard-B in most new peripheral designs.