Explain the difference between a change in quantity demanded and a change in demand.

Change in quantity demanded" refers to movement with the demand curve.  For instance, if the price of apples rises, all other things being equal, and people will buy fewer apples; thus, the quantity demanded will reduces.  A "change in demand" refers to a situation in which the whole demand curve shifts.  For example, if a large number of new people move into your neighbourhood, there will be a larger pool of people interested in buying apples at the local grocery store.