لخّصلي

Functions of money:
Whether money is shells or rocks or gold or paper, it has three primary functions in any economy: as a medium of exchange, as a unit of account, and as a store of value. Of the three functions, its function as a medium of exchange is what distinguishes money from other assets such as stocks, bonds, and houses.
Medium of Exchange
In almost all market transactions in our economy, money in the form of currency or checks is a medium of exchange; it is used to pay for goods and services. The use of money as a medium of exchange promotes economic efficiency by minimizing the time spent in exchanging goods and services. To see why, let's look at a barter economy, one without money, in which goods and services are exchanged directly for other goods and services.
Take the case of Ellen the Economics Professor, who can do just one thing well: give brilliant economics lectures. In a barter economy, if Ellen wants to eat, she must find a farmer who not only produces the food she likes but also wants to learn econom-ics. As you might expect, this search will be difficult and time-consuming, and Ellen might spend more time looking for such an economics-hungry farmer than she will teaching. It is even possible that she will have to quit lecturing and go into farming herself. Even so, she may still starve to death.
The time spent trying to exchange goods or services is called a transaction cost. In a barter economy, transaction costs are high because people have to satisfy a "double coincidence of wants —they have to find someone who has a good or service they want and who also wants the good or service they have to offer.
Let's see what happens if we introduce money into Ellen the Economics Professor's world. Ellen can teach anyone who is willing to pay money to hear her lecture. She can then go to any farmer (or his representative at the supermarket) and buy the food she needs with the money she has been paid. The problem of the double coincidence of wants is avoided, and Ellen saves a lot of time, which she may spend doing what she does best: teaching.
As this example shows, money promotes economic efficiency by eliminating much of the time spent exchanging goods and services. It also promotes efficiency by allowing people to specialize in what they do best. Money is therefore essential in an economy:
It is a lubricant that allows the economy to run more smoothly by lowering transaction costs, thereby encouraging specialization and division of labor.
The need for money is so strong that almost every society beyond the most primitive invents it. For a commodity to function effectively as money, it has to meet several criteria: (1) It must be easily standardized, making it simple to ascertain its value;
(2) it must be widely accepted; (3) it must be divisible, so that it is easy to "make change";
(4) it must be easy to carry; and (5) it must not deteriorate quickly. Objects that have satisfied these criteria have taken many unusual forms throughout human history, ranging from wampum (strings of beads) used by Native Americans; to tobacco and whiskey, used by the early American colonists; to cigarettes, used in prisoner-ot-war camps during World War Il.' The diverse forms of money that have been developed over the years are as much a testament to the inventiveness of the human race as are the developments of tools and language.
Unit of Account
The second role of money is to provide a unit of account; that is, money is used to measure value in an economy. We measure the value of goods and services in terms of money, just as we measure weight in terms of pounds or distance in terms of miles. To see why this function is important, let's look again at a barter economy, in which money does not perform this function. If the economy has only three goods-say, peaches, economics lectures, and movies— then we need to know only three prices to tell us how to exchange one for another: the price of peaches in terms of economics lectures (that is, how many economics lectures you have to pay for a peach), the price of peaches in terms of movies, and the price of economics lectures in terms of movies. If there were 10 goods, we would need to know 45 prices in order to exchange one good for another; with 100 prieds, we would need 4, 950 prices; and with 1,000 goods, we would need Imagine how hard it would be in a barter economy to shop at a supermarket with 1,000 ditterent items on its shelves and be faced with deciding whether chicken or fish is a better buy if the price of a pound of chicken were quoted as 4 pounds of butter and the price of a pound of fish as 8 pounds of tomatoes. To make it possible to compare prices, the tag on each item would have to list up to 999 different prices, and the time spent reading them would result in very high transaction costs.