The demand curve is downward-sloping and relates quantity demanded (horizontal axis) to price (vertical axis). The demand curve acquires its negative slope from the law of demand, which states that when the price of commodity is raised (other factors being held constant), buyers tend to buy less of the commodity (presuming that the commodity is a normal good). On the other hand, when the price is lowered, other things being constant, quantity demanded increases.

FIGURE - Demand
Demand slopes downward, showing that price and quantity demanded are inversely related. For example, as the price drops from $4 to $1, the quantity demanded rises from 1 unit to 4 units. This is seen as a movement from point B to point E along the demand curve.