Demand vs Quantity Demanded

When talking about Demand and Supply it is important to differentiate between the terms Demand (or Supply) and Quantity Demanded (or Quantity Supplied). While they sound the same, they are not interchangeable.

When we use the term Demand we are referring to changes in the entire demand curve. While when we refer to Quantity Demanded we are referring to a movement along the demand curve.

Remember the Law of Demand states that certis pariubs (economic speak for other things being equal) a change in price will result in quantity demanded changing in the opposite direction. Or as your book (Economics Today, The Micro View, and the Macro View, 14th Edition, by Roger LeRoy Miller, 2008, Pearson Education, Inc) in Chapter 3, page 52 states:

When the price of a good goes up, people buy less of it, other things being equal. When the price of a good goes down people buy more of it, other things being equal.

The Law of Demand deals exclusively with changes in price and quantity and, because of this, when we are describing changes under the Law of Demand we have to prefix the word Demand with the word Quantity. Thus, we refer to a change in Quantity Demanded rather than a change in Demand which refers to some other factor (income, tastes, changes in compliments and substitutes, etc.) changing.

While some will dismiss this as merely an exercise in semantics, it is important and will lead to problems in both understanding what is going on with demand and supply as well as lead to wrong answers on tests.