When I'm standing in my room on the 10th floor, is my body at the same voltage potential as the ground pin? I don't think this is the case, because the ground wire doesn't make contact with the floor I'm standing
A building has usually rather low ground resistance. Think of all the plumbing and other metal structures, for example.
So probably the 10th floor is at 110V and the ground pin is at 0V, and when I touch the ground pin I get a shock.
If the ground resistance is (unusually) high, you won't get a lethal shock, because there will be no current flow. Note that you may get a small shock from your own electrostatic discharge, but that is not dangerous.
Where is that quote from? It is self-contradictory, and pretty much just plain wrong.
Current will flow if a load is connected between a generic power supply positive and negative, no ground connection is needed, unless you have some special purpose power supply.
A ground connection is often used for safety, especially in line non-isolated supplies, or as a means to reduce noise. Even so, not all supplies pass the ground on to the output terminals or even make it available (for common examples, think wall-warts and such with just a two pole output). Many supplies don't even have an incoming ground terminal from the mains (small switchers, and again, wall-warts).
I was going to post this as a comment, but it seemed to cover the question.
Edit as per comment below and corrected text.
Well, that does change things. Generally, a DC power supply has no output tied to ground, or connected to the supply electrically at all, so connecting a load between an output and earth ground would result in no current flow. Supplies that have an output set of terminals and a separate ground can often be configured (by jumping ground to one of the output terminals) as positive ground or negative ground if desired.
Best Answer
The main point of grounding a line-powered appliance is to electrically "box up" the dangerous parts. If, for example, a "hot" wire comes loose inside the appliance and touches the metal case, the current will flow thru the ground connection to that case. That will blow a fuse, trip a breaker, or trigger the ground fault interruptor if that line is equipped with one.
If the case weren't grounded, then the same loose wire now puts the case at the hot potential. If you come along and touch it and something else grounded, like a faucet, at the same time, the full 220 V is now applied across your body.
You are right that touching just a hot wire without touching anything else conductive won't hurt you. Presumably the "tile" floor you are talking about is made of insulating material. However, the reason this is unsafe is that often you are not completely insulated from everything else. If you touch the faulty appliance and happen to brush against a water faucet, the case of your desktop computer, a radiator, or any other appliance that is ground, you can be seriously hurt. Even a concrete floor can carry enough current to be dangerous.