Economics Online Tutor
Basic definitions relating to demand:

The quantity of a specific good or service that people are willing and
able to purchase decreases as the price increases, and increases as
the price decreases, as long as the price is the only thing that
changes.

You may see the law of demand defined using slightly different words, but the
meaning is the same.  Implied in the definition is a specific time frame.  The word
"quantity" does not have much relevance unless a specific time frame is involved.
However, just knowing that a time frame is involved, and not having to know what
the specific time frame happens to be, will be sufficient for most kinds of analysis.
So the time frame remains implied, and not repeatedly stated during the analysis.
Also, it is important to note, before moving on, the fact that this definition states
that price and quantity demanded move in opposite directions: when one
increases, the other decreases.  This is called a negative correlation.

The law of demand requires an understanding of the definitions of two more terms:
demand, and the quantity demanded.

Demand: The amount of a specific good or service that people are willing and able
to purchase at every possible price.

Quantity demanded: The amount of a specific good or service that people are
willing and able to purchase at one specific price.

These two definitions (demand, quantity demanded) form a distinction that is very
important in the study of economics.  This distinction also happens to be one that
many students have trouble understanding.  For this reason, I have included a
more detailed description of this distinction below.
 Demand
Now, on to more definitions:

Demand schedule: A list of prices and their corresponding quantities demanded.

Demand curve: A graph of the demand schedule.  Often a curve turns out to be a straight line, but in
economics it is still referred to as a curve.  By tradition, the demand curve is a graph with an origin of
(0,0), quantity on the horizontal (X) axis, and price on the vertical (Y) axis.  Some students of
mathematics ask why this appears to be inconsistent with the concept of plotting independent variables
on the X-axis.  The only answer that I have been able to find is tradition.  The demand curve slopes
downward because of the negative correlation in the law of demand.

Market demand curve: The sum of all individual demand curves in a given market.

Finally, a couple more definitions, along with the promised additional explanation of the distinction
between demand and quantity demanded:

Change in demand: When one of the determinants of demand changes, demand changes.  The entire
demand schedule, or demand curve, changes.  This means that the quantities demanded at every price
change.  This is shown by a shift in the demand curve.  If demand increases, the demand curve shifts to
the right (or up, depending on what terminology you use).  If demand decreases, the demand curve
shifts to the left (or down).

Change in the quantity demanded: If the price of the good or service in question (the good or service
that is used to plot the demand curve) is the only thing that changes, then demand does not change.
The demand curve does not change (or shift), but the price / quantity combination moves to a different
point on the existing demand curve.

As stated above, many students have difficulty grasping the distinction between demand and quantity
demanded.  The first thing to do in trying to understand this is to look at the difference in the definitions of
the two terms.  Demand refers to every possible price; quantity demanded refers to a specific price.  If the
price changes, that change is already covered by demand; demand includes all prices.  So a change in price
will not change demand, it will only involve a movement along an existing demand curve.  That would be
called a change in the quantity demanded.  In order for demand to change, there must be a change in a
non-price factor of demand (one of the determinants of demand).  In that case, the quantity demanded will
change at every price, and the demand curve will shift as a result.

Another way to look at it, that may be helpful to some students: if the change is plotted on an axis (price is
plotted on the vertical axis), then the change is a change in the quantity demanded.  If the change is not
plotted on an axis (the determinants of demand are not listed on an axis), then the change is a change in
demand.

This analysis of supply & demand continues with definitions relating to supply.    CLICK HERE TO
GO DIRECTLY TO THE SECTION ON SUPPLY.
But first, a few more definitions of terms that are relevant to demand,
and the distinction between demand and quantity demanded:

Factors of demand: Factors of demand are the price of the good in question,
plus other things that determine the level of demand.  You will find that an
analysis of demand will require the price to be considered separately from
the other things that determine demand.  The other things are called
determinants of demand.

Determinants of demand: Consumer income, consumer tastes, the prices of
complements, the prices of substitutes (you may find the prices of
complements and substitutes lumped together in a category called 'the
prices of related goods'), consumer expectations, and the number of
potential buyers.  A more detailed definition of each determinant of demand
follows:

Consumer income: The more income that people have, the higher the
demand for goods and service in general.  This means that for each specific
normal good, income and demand are positively correlated: income and
quantity demanded change in the same direction.  An inferior good is the
opposite of a normal good.  An inferior good is one that people buy less of if
their incomes increase.  They choose to use the higher income on more
expensive substitutes, and buy fewer of the inferior good.  An inferior good
is an exception to the law of demand that states that a negative correlation
between price and quantity demanded exists.

Consumer tastes: When consumer tastes change, the demand for specific
goods and services also changes.  Styles come and go, fads come and go.

Prices of complements: Complements are two goods that consumers tend to
purchase together.  Since consumers purchase two items as if they were
one (a package deal), if the price of one of the items changes, so will the
price of the package deal, and the demand for the other item will change
accordingly.  If the price of one good increases, the demand for its
complement decreases, and vice versa (negative correlation).

Prices of substitutes: Substitutes are two goods that, in the eyes of
consumers, can be substituted for each other.  If they spend their income on
one, they won't want to spend their income on the other.  They may prefer
one to the other, but if the price of one changes, it may influence which one
they prefer at the different relative price.  If the price of a good or service
increases, the demand for its substitute increases, and vice versa (positive
correlation).

Consumer expectations: If consumers believe that prices will change in the
future, or their incomes will change in the future, it may affect how much of a
given good or service they purchase now, instead of later.

Number of potential buyers: Since market demand is equal to the sum of all
individual demands, the number of potential buyers will influence the total
demand for a good or service.