Law of Demand states that other things being constant, amount demanded falls with rise in price and rises with a fall in price. Here two things are worth discussing. First one is price and second one is other factors other than price.
Demand for a commodity depends on several factors like price, income, taste and preference of consumers, price of other related commodities, future expectations of price and population etc. Changes in demand imply two things: (i) change in quantity demanded (ii) change in demand.
(1) Change in quantity demanded:
Change in quantity demanded refers to the change in the amount of a commodity as a result of change in the price of it. Amount demanded rises or falls according to the fall or rise in price. In such a case other factors influencing demand are held constant. The fall and rise in amount demanded due to the change in price is technically called "contraction" and "extension" of demand.
The demand function or the demand curve never changes. The change takes place in the same demand curve. The existing demand curve contains the changes in the different price-quantity combination. In case of change in quantity demanded movement takes place along the existing demand curve.
(2) Change in demand:
'Change in demand' means changes in demand due to the change in the factors other than price. Those other factors are income, taste and preference, population, future expectation, prices of other related commodities etc. Price remaining constant these factors bring about a change in demand which is called "Change in demand". The change in demand involves "increase" and "decrease" of the demand for a commodity.
The change in demand implies a change in the demand function itself. In case of change in demand the entire demand schedule and demand curve change. With an increase in demand curve shifts upward and with a decrease in demand curve shifts downward. Thus change in demand takes place on different demand curves.