Final answer:
A change in demand involves a shift of the entire demand curve due to external factors, while a change in quantity demanded is a movement along the demand curve resulting from a variation in price.
Step-by-step explanation:
The primary difference between a change in demand and a change in quantity demanded relates to factors that influence demand as a whole versus changes in the quantity demanded due to a change in price. A shift in demand occurs when, at any given price, the quantity demanded changes due to external factors such as income increase, changes in tastes, or prices of related goods. This is represented graphically as a shift to the left or right of the entire demand curve. In contrast, a change in quantity demanded is a movement along the same demand curve which occurs solely because of a change in the price of the product itself. At the upper end of a demand curve, where the price is high and the quantity demanded is low, a small change in the quantity demanded represents a significant percentage change.