Supply vs. Demand

Anyhoos, I was reading reddit and some article on current affairs in the U.S and the following thought came to me;

Economics is based on supply and demand, correct?
So the question I want answered is, does demand include the ability to purchase or is it simply a want? Same goes with supply, does it include the ability to actually sell a good/service and not simply based on speculation?

The above questions came when I was reading about the rich/poor divide in the U.S and got to thinking, if there were only two people in the world, one super rich, the other very poor. Does the 'wealth' of the super rich guy equate to nothing if the poor guy cant afford what the rich guy has, or better yet, has no desire in getting his wealth?

Just some random thoughts....