There are, I guess legacy, 2 wire systems. On those systems, one wire is hot and the other is neutral. The neutral is connected to ground at the electrical box.
For 3 wire systems, there is an additional ground wire. It comes into play when the hot wire shorts to a part of the device that is exposed to the environment (hopefully tripping the breaker).
Modern 2 pole plugs are constructed with different size pins, so a device can be wired to be safer when connected to a properly wired 2 pole outlet.
For 3 wire systems, there is an additional ground wire. It comes into play when the hot wire shorts to a part of the device that is exposed to the environment (hopefully tripping the breaker).
Modern 2 pole plugs are constructed with different size pins, so a device can be wired to be safer when connected to a properly wired 2 pole outlet.