When someone want a thing and also have ability(resource) to buy is called demand when someone want a thing but does not has ability(resources) to buy is called desire
Desire is a imagination of without estimating the available resources and demand is a thing which is based on the availability and ability to consumption of resources.
"A commodity has a demand if a person is willing and has the ability to buy it". Three kinds of demands may be distinguished: (a) Price Demand; (b) Income Demand; and (c) Cross Demand. (a) Price Demand: Price demand refers to the various quantities of a commodity or service that a consumer would purchase at a given time in a market at various hypothetical prices. It is assumed that other thing, such as consumer's income, his tastes and prices of inter-related goods remain unchanged.
(b) Income Demand: The income demand refers to the various quantities of goods and services which would be purchased by the consumers at various levels of incomes. Here it is assumed that the price of the commodity or service as well as the price of inter related goods and the tastes and desires of consumers does not change. Just as the price demand expresses relationship between prices and quantities, the income demand shows the relationship between income and quantities demanded.
(c) Cross Demand: The cross demand means the quantities of a good or service which will be purchased with reference to change in price not of this good but of other inter-related goods. These goods are either substitutes or complementary goods. A change in the price of tea, for instance, will affect the demand for coffee. Similarly, if horses become cheap, demand for carriages may increase.
Demand means a desire for a thing backed by purchasing power and willingness to use the same for purchasing it. Where desire for a thing does not constitute demand.
A request doesn't have a penalty attached for refusal. A demand is couched in terms of "do it, or else" whether explicitly stated or not, there's a penalty attached to refusing.
