At a certain time, when other factors affected the demand remains constant, that is, quantity demanded decrease when price is increased, and quantity demande  increased when price decreased.
• Due to this inverse (negative) relationship between the price of the good and the
quantity demanded, the point moves along the demand curve.
• When moving upwards along the curve, the demand is contracted and when moving
downwards along the curve, the demand is expanded.
• When the price of the good remains constant, the demand curve is shifted right or
left due to other factors: price of related goods, taste of the consumer and future
price expectations.
• These are known as the decrease in the demand and increase in the demand.
• The change in quantity demanded and the change in demand are two concepts which
are different from each other.