What is difference between demand and quantity demanded?

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.