20mA current loop was the old serial signaling scheme (RS232 on ASR33 teletypes come to mind).
4-20mA is an analog signal where the output of a sensor varies between 4mA and 20mA. Current loops are often used when you have long cables since as long as it is constant and not too high, the series resistance of the cable is not important. If you have a voltage source, the series resistance would form a voltage divider with the input impedance of the receiver/meter.
