A demand curve is a graphical representation that shows the relationship between the price of a good or service and the quantity demanded by consumers. Typically, it slopes downward from left to right, indicating that as prices decrease, the quantity demanded increases. This reflects the basic economic principle that consumers are more likely to buy more of a product when it is cheaper. The demand curve can shift due to various factors, such as changes in consumer preferences, income levels, or the prices of related goods. For example, if the price of a substitute good, like butter, rises, the demand for margarine may increase, shifting its demand curve to the right.