Final answer:
When quantity demanded decreases in response to its own price change, there is a movement up along the demand curve, whereas a decrease in overall demand would shift the entire demand curve to the left.
Step-by-step explanation:
When quantity demanded decreases in response to a change in its own price, this indicates that there is a movement up along the demand curve. This movement is not to be confused with a shift of the demand curve itself, which is a different concept. When the entire demand curve shifts due to changes in factors other than the price of the product itself, it may either shift to the right, indicating an increase, or to the left, indicating a decrease in demand across all price points. In your case, a decrease in demand would shift the demand curve to the left, illustrating that there would be fewer quantities demanded at each price level.