The difference between a change in demand and a change in quantity demanded is that the first is a movement in the entire demand curve while the second is a movement along a given demand curve.  In addition, a change in quantity demanded is caused by a change in the price of the good or service while a change in demand must be caused by something else.

Demand can be defined as the amount of a good or service that consumers are willing and able to buy at a range of prices.  In other words, demand must be represented by a curve.  It must show how much people will buy at each price point.  What this means is that a change in the price of the good or service cannot cause a change in demand.  All that will happen if the price changes is that there will be a different quantity demanded. In order to change the actual demand for something, something has to happen to change how much people are willing or able to buy at a given price.  For example, if people get richer, they will be able to buy more of a good even though the price does not change.  As another example, when people hear about the scandal with Volkswagen diesel cars, they will be less willing to buy those cars even if the price does not change.

Changes in demand are caused by things other than a change in the price of the product.  They are represented by a shift in a demand curve.  Changes in quantity demanded are only caused by changes in price.  They are represented by movement along a given demand curve.