The demand is for various quantities of goods which are ready to be purchased at various prices when other factors affecting demand remain constant.

Demand can be categorized as follows.

• Individual demand
• Market demand

The individual demand is for various quantities of goods, buyers are ready to buy at various prices at a certain time period.

The market demand is a sum of various quantities of goods which are ready to be bought at various prices by all buyers in a market for a certain good, at a certain time period.

The theory of demand is analyze the changes of demand according to changes of the determinants of demand.

The factors mentioned below affect determine the market demand.

• The key determinants of demand can be listed as below.
• Price of the good concerned (P)
• Prices of related goods (Substitutes and complementary goods) (Pn)
• Consumer income (Y)
• Consumer taste (T)
• Future expectations (Ex)
• Number of buyers and its composition. (N)

That, it is possible to show the relationship between the individual demand for any good and its determinants, as an equation given below.

Qdx = F (P, Pn,Y,T,Ex)

The relationship shown by this is named “individual demand function”.

That, it is possible to show the relationship between the market demand for any good and its determinants, as an equation given below.

Qdx = F (P, Pn, Y, T, Ex, N)
That, the relationship shown by this is named “Market demand function”.