In economics the demand curve is the graph describing the relationship between the price of a certain commodity and the amount of it that consumers are willing and able to purchase at that given price. It is a graphic representation of a demand schedule. The demand curve for all consumers jointly follows from the demand curve is very specific consumer: the individual demand at each price are added together.
An example of a demand curve shifting. The shift from D1 To D2 means an increase in demand with consequences for the other variables
Demand curve are used to eliminate behaviors in competitive markets and are often combined with supply curves to estimate the equilibrium price and the equilibrium quantity of the market. In a monopolistic market, the demand curve facing a monopolist is simply the market demand curve.