The difference between demand and quantity demanded lies primarily in their scope and what causes their change:

Demand

• Demand refers to the entire relationship between the price of a good or service and the quantities consumers are willing and able to buy at those different prices. It is represented by the whole demand curve on a graph, which plots various quantities demanded at various prices

• Demand reflects the willingness and ability of buyers to purchase a good or service at all possible prices during a given period.
• Changes in demand occur when factors other than the good's own price change, such as consumer income, tastes, prices of related goods, number of buyers, or future expectations. This causes the entire demand curve to shift either rightward (increase in demand) or leftward (decrease in demand)

Quantity Demanded

• Quantity demanded is the specific amount of a good or service that consumers are willing and able to purchase at a particular price, at a given point in time

• It is represented by a single point on the demand curve corresponding to one price and one quantity

• Changes in quantity demanded are caused only by changes in the price of the good itself, resulting in movement along the same demand curve (either upward or downward). For example, a price decrease leads to an increase in quantity demanded, and a price increase leads to a decrease in quantity demanded, reflecting the law of demand

Summary Table

Aspect| Demand| Quantity Demanded
---|---|---
Definition| Relationship between price and quantity over all prices| Specific quantity at a single price
Graphical Representation| Entire demand curve| A single point on the demand curve
Cause of Change| Changes in non-price factors (income, preferences, related goods)| Change in price of the good
Effect of Change| Shift of the entire demand curve (right or left)| Movement along the demand curve

In essence, demand is the whole curve showing how quantity demanded varies with price, while quantity demanded is a particular quantity at a specific price on that curve