A change in quantity demanded occurs because of a change in the price of the product. It is indicated by a movement on the same demand curve. There is an inverse relationship between quantity demanded and price of a product while other factors remain constant. So when price increases, the quantity demanded falls and vice versa.
A change in demand occurs because of a change in other factors while the price of the product remains constant. This is indicated by a shift in the demand curve. The other factors are the price of other goods, tastes, and preferences, the income of the consumer, etc.
