Eccezan's Wikipidia quote failed to include the preceding and explanatory paragraph.
I want to add some information to make the issue slightly clearer (good luck if you know nothing about economics).
http://en.wikipedia.org/wiki/Supply_...and#Criticisms
The rest of the quote.
"Supply and demand is an economic model of price determination in a market. It concludes that in a
competitive market, the unit price for a particular good will vary until it settles at a point where the quantity demanded by consumers (at current price) will equal the quantity supplied by producers (at current price), resulting in an economic equilibrium of price and quantity."
Competitive Markets:
http://en.wikipedia.org/wiki/Perfect_competition
In economic theory, perfect competition describes markets such that no participants are large enough to have the market power to set the price of a homogeneous product. Because the conditions for perfect competition are strict, there are few if any perfectly competitive markets. Still, buyers and sellers in some auction-type markets, say for commodities or some financial assets, may approximate the concept.