What Demand Is:
Economists have a very precise definition of demand. For them demand is the relationship between the quantity of a good or service consumers will purchase and the price charged for that good. More precisely and formally the Economics Glossary
as "the want or desire to possess a good or service with the necessary goods, services, or financial instruments necessary to make a legal transaction for those goods or services."
What Demand Is Not:
Demand is not simply a quantity consumers wish to purchase such as '5 oranges' or '17 shares of Microsoft', because demand represents the entire relationship between quantity desired of a good and all possible prices charged for that good. The specific quantity desired for a good at a given price is known as the quantity demanded. Typically a time period is also given when describing quantity demanded.
Demand - Examples of Quantity Demanded:
When the price of an orange is 65 cents the quantity demanded is 300 oranges a week.
If the local Starbucks lowers their price of a tall coffee from $1.75 to $1.65, the quantity demanded will rise from 45 coffees an hour to 48 coffees an hour.
A demand schedule is a table which lists the possible prices for a good and service and the associated quantity demanded. The demand schedule for oranges could look (in part) as follows:
75 cents - 270 oranges a week
70 cents - 300 oranges a week
65 cents - 320 oranges a week
60 cents - 400 oranges a week
A demand curve is simply a demand schedule presented in graphical form. The standard presentation of a demand curve has price given on the Y-axis and quantity demanded on the X-axis.
The Law of Demand:
The law of demand states that, ceteribus paribus
(latin for 'assuming all else is held constant'), the quantity demanded for a good rises as the price falls. In other words, the quantity demanded and price are inversely related. Demand curves are drawn as 'downard sloping' due to this inverse relationship between price and quantity demanded.
Price Elasticity of Demand:
The price elasticity of demand represents how sensitive quantity demanded is to changes in price. Further information is given in the article Price Elasticity of Demand