Understanding the factors that influence demand is crucial in economics. Demand can change due to various determinants, but it's important to note that a change in price does not shift the demand curve; instead, it results in a movement along the existing curve. This distinction is vital for grasping how demand operates in different scenarios.

When analyzing demand, we typically label our axes with price (P) on the vertical axis and quantity (Q) on the horizontal axis. For example, if we start at a point labeled (P1, Q1) and the price increases to P2, the quantity demanded will decrease, moving to a new point (P2, Q2) on the same demand curve. This scenario illustrates a decrease in quantity demanded due to the price increase, while the demand curve itself remains unchanged.

In contrast, demand can also shift due to other determinants, such as consumer income or preferences. For instance, if consumer preferences change and they begin to favor a particular good more, the demand curve will shift to the right, indicating an increase in demand. In this case, even if the price remains constant at P1, the quantity demanded at that price will increase, moving to a new point on the new demand curve (D2). This situation highlights an increase in demand rather than a change in quantity demanded.

To summarize, a change in price results in a movement along the demand curve, affecting the quantity demanded, while changes in other determinants lead to a shift in the demand curve itself. This distinction is essential for understanding how various factors influence consumer behavior and market dynamics. Remember, the principle of ceteris paribus, which means "all else being equal," applies in both scenarios, ensuring that only one variable is changing at a time.