A USB A-to-B cable is a cable that is meant to be plugged in only one way and features two different connectors to make it impossible to connect incorrectly. USB A is the upstream port and USB B is the downstream port.
The USB A port connects to the computer and the USB B port connects to the device. The USB A port is rectangular in shape and the USB B port is square with two cut corners to ensure correct orientation.
USB is a high speed bus that allows a vast array of devices to connect to the same type of port. This eliminates many of the complexities of adding peripherals to computers before USB ports became standard.