[originalfacsimi]

I don't know why but ever site I visit someone will bring up this subject as well as it should be; personal safety.

The electric company provides the main ground at all locations. Easy to see are the pole mounted Xformers as you can see the main ground running down the side of the pole. This is to protect THEIR equipment. As you enter your service your MUST also have a grounding rod and this is to protect your equipment, not so much as you. Bill is correct when he talked about the two prong plug. The neutral and the ground are bonded at the service panel so that if you lose either of these two wires (neutral/ground) you will have protection.
Now if you have a hot tub, whirlpool, under elevation connections (washer/dryer in basement) you will need a GFCI which is for personnel protection. In this instance there must be three separate wires run with nothing bonded. These circuits work on a difference of current entering a circuit verus current leaving. If any difference they trip.
Now as everone has stated, you must not create a ground loop particually at these extremely low signals. Most of the new homes take care of the grounding of the house is bounded/welded physically together however in the older homes we need to take care.
Your service panel is to be grounded to a ground rod (depth determined by local ordinace) and in order to not form a ground loop, everything else (externally) should be grounded to SAME.