In economics, we need to use terms a little more carefully than they are sometimes used in ordinary discussions. In general, "Demand" is a word that can have more than one meaning, but in microeconomics we define it more carefully so that it has only one meaning.
Demand is the relationship between price and quantity demanded for a particular good and service in particular circumstances. For each price the demand relationship tells the quantity the buyers want to buy at that corresponding price. The quantity the buyers want to buy at a particular price is called the Quantity Demanded.
The reason demand is defined in such a way is to keep it simple, we may think of the buyers as consumers. Clearly, the buyers are the people who want or need the product or service -- but there is more to it than that. The word "demand" refers to the willingness and ability of people to purchase the good or service in the market. The demand relationship expresses that willingness and ability for the whole range of prices. To say that a person has a demand for a particular product is to say that the person has money with which to buy and is willing to exchange the money for the good. People will not demand what they do not want or need, but a want or a need not backed by the purchasing power is not a demand. .
Similarly, it is not enough that the suppliers possess the good or (the capacity to perform) the service. Supply also means willingness to sell. .
Most of us have experience living in the market economic system, and that makes economics seem like a common-sense field -- but sometimes that common-sense feel can be deceptive. People sometimes use the term "demand" ambiguously -- as if "demand" were the same thing as need. But it is not a need without purchasing power will not create effective demand in the marketplace. Economists sometimes stress this point by using the term "effective demand" in place of simple "demand.
