More on 3-Prong Plugs.
What has been said is correct, but if one follows the wires back into
the fuse (breaker) box, one will find that the earth ground (green) and
the electrical company ground (white) will (usually ??) go to the same
terminals.
So, then it would seem that one reason for the third prong on the
3-Prong Plug is to allow it to be plugged in in only one way --
polarized.