Change in Quantity Demanded:
This is a situation which is due to a rise or fall in the price of a commodity/service and it is described by a movement along the demand curve for the commodity/service.
Fig. 1: Movement induced by an increase in commodity’s own price
Fig. 2: Movement due to a decrease in commodity’s own price
Change in Demand:
This is a situation which is due to a variation in any determinant of demand other than commodity/service own price and it is described by a shift in the demand curve for the commodity or service.