Concept of Demand?
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Demand is defined as the quantity of a good or service that consumers are willing and able to buy at a given price in a given time period.

It is necessary to distinguish between demand and desire or need. Demand in economics must be effective. It means that when a consumers` desire to buy a product is backed up by an ability and willingness to pay for it, then only it actually affect the market. If a person below poverty line wants to buy a car, it is only a desire but not a demand as he cannot pay for the car. If a rich man wants to buy a car, it is demand as he is able and willing to pay for the car. Thus, desire backed by willingness and ability to pay is called demand.