A change in the price of a good or service causes a movement along the demand curve.
A price rise will cause a decrease (contraction) in the quantity demanded of the product.
A rise in price from P2 to P1 causes demand to fall from Q2 to Q1.
A reduction in price will cause an increase (expansion) in the quantity demanded.
A fall in price from P2 to P3 causes demand to expand from Q2 to Q3.