Doty, R. G., and Stanley Fischer. "Money." World Book Online Reference Center. 2005. World Book, Inc. 26 Oct. 2005. http://www.worldbookonline.com/wb/Article?id=ar367640

"Money is anything that is generally accepted by people in exchange for the things they sell or the work they do. Gold and silver were once the most common forms of money. But today, money consists mainly of paper bills, coins made of various metals, and checking account deposits.

Money has three main uses. First, and most important, it is a medium of exchange—that is, something people will accept for their goods or services. Without a medium of exchange, people would have to trade their goods or services directly for other goods or services. If you wanted a bicycle, you would have to find a bicycle owner willing to trade. Suppose the bicycle owner wanted skis in exchange for the bike and you did not own skis. You would then have to find something a ski owner or ski maker wanted and trade it for skis to give the bicycle owner. Such trading, called barter, can take much time. A modern, industrialized country could not function without a medium of exchange.

A second use of money is that it serves as a unit of account. People state the price of goods and services in terms of money. In the United States, people use dollars to specify price, just as they use hours to express time and miles or kilometers to measure distance.

A third use of money is as a store of wealth. People can save money and then use it to make purchases in the future. Other stores of wealth include gold, jewels, paintings, real estate, and stocks and bonds.

Any object or substance that serves as a medium of exchange, a unit of account, and a store of wealth is money. To be convenient, however, money should have several qualities. It should come in pieces of standard value so that it does not have to be weighed or measured every time it is used. It should be easy to carry so that people can carry enough money to buy what they need. Finally, it should divide into units so that people can make small purchases and receive change.

In the past, people used beads, cocoa beans, salt, shells, stones, tobacco, and other things as money. But above all, they used such metals as copper, gold, and silver. These metals made convenient, durable money.

Today, most money consists of paper. The paper itself is of little value, but it is accepted in exchange. People accept pieces of metal or paper in exchange for work or goods for only one reason: They know that others will take the same metal or paper in exchange for the things they want. The value of money therefore results from the fact that everyone will accept it as payment"

Comments: This tells me that back then gold and silver were the form of trade. It also tells me that money is mostly the modern form of trade.