Demand is the quantity a buyer is willing and able to sell at a given price in a given period of time.

When we plot the demand curve, price is on the x axis and quantity is on the y axis. The demand curve is upward sloping. This is because there is an inverse relationship between price and quantity i.e. as price rises quantity falls.

A movement along the demand curve is caused by a change in the price of the good. A decrease in price causing a rise in quantity demanded will cause a contraction in the demand curve.

Other factors that cause quantity demanded to rise or fall make the demand curve shift. If the demand curve shifts to the left quantity demanded rises.

Factors that cause the demand curve to shift include price of substitutes, advertising, indirect taxes and income.