Consider the following events: The price of cell phones goes down by 25 percent during a sale. This event would cause a movement along the demand curve, rather than a shift in the demand curve.
When the price of a good changes but other factors remain constant, it results in a movement along the demand curve. This is termed as a "change in quantity demanded."
So, if the price of cell phones goes down by 25 percent during a sale:
Increase in Quantity Demanded: There will be an increase in the quantity demanded for cell phones. At the lower price, more consumers will be willing and able to buy cell phones.
Movement Along the Curve: This change is represented by a movement from one point to another along the demand curve, moving downward (from a higher point to a lower point) to represent the increased quantity demanded at the lower price.
On the other hand, shifts in the demand curve are caused by non-price factors, such as changes in consumer preferences, income levels, prices of related goods (substitutes or complements), expectations about future prices, population or demographics, and so on. If one of these factors changes, then the entire demand curve would shift either to the right (increase in demand) or to the left (decrease in demand).