The law of demand:
When the Px increases, the quantity of x demanded decreases.
When the Px decreases, the quantity of x demanded increases.

Two explanations for the law of demand:
(1)  Ability:  when the price of x goes up, real income decreases, ceteris paribus, and the consumer buys less of x.
(2)  Willingness:  the law of diminishing marginal utility (be able to explain)
 

The demand curve or schedule:
A schedule showing the amount of a good that consumers are willing and able to buy at various prices during a specific period of time.

The demand function:

Qx demanded = f( Px, Income, Price of substitutes, Price of complements, price and income expectations, time, taste)

Income:
    Normal good = when income increases, consumers buy more of the good.
    Inferior good = when income increases, consumers buy less of the good.

Substitutes:
    Consumers always substitute into the relatively cheaper good.

Complements:
    Consumers buy both goods together.  They "complement" each other.
 

The Demand Curve (Graphically):

When the Px changes, we move along with curve.
When any other independent variable changes, we shift the curve.