Dear student,
When there is a change in the units of a commodity demanded due to a change in the price of the commodity we say that there is a change in the quantity demanded. On the other hand, when there is a change in the units of a commodity due to a change in the factors other than price we say that there is a change in the demand.
It must, however, be noted that a demand function includes both price as well as other factors as determinant of demand for a commodity.
Regards