The graph drawn with the the quantity of a product or service demanded represented on the x-axis and the price for the product or service represented on the y-axis is called the demand. Now, the standard term demand in economics refers to the complete graph. Demand changes due to many factors some of which could be a change in technology, change in the taste of the consumer, etc. When there is a change in demand the entire graph shifts.
Quantity demanded refers to the value on a single point of the demand graph. This is dependent on the price. For every value of price there is a corresponding value of demand on the demand curve. If one looks at the different points on the demand curve the quantity demanded for each price can be determined.
In economics demand refers in general to the quantity of a good or service that buyers want to purchase. In order to explain clearly the nature of behavior of this demand, economist use different associated terms such as demand curve, and quantity demanded. In particular, economist differentiate between changes in demand because of changes in price, for that due to other reasons. Thus they speak of a demand curve that shows quantity of goods that buyers would purchase at different prices. Quantity demanded refers to the quantity corresponding to a given price as per the demand curve. Thus the quantity demanded is the quantity that buyers want to purchase at a given price as per a demand curve.
However the shape or nature of the demand curve itself changes depending on factors like change in income and preferences of people. Such change in the nature of demand curve are called a change in demand. Used this way, demand refers to the quantities buyers are prepared to purchase at different prices when all other factors are held constant.