Economic Properties of Money
Money is defined as something which serves as a means of exchange, a store of value, and a unit of account. The currency which serves as money may be backed by some tangible commodity, or may be fiat money. Alternatively, trade may be carried on using commodity pay, wherein existing commodities are assigned exchange values but not explicitly designated as money.
Money Neutrality and the Money Market
Money may be viewed as neutral or non-neutral regarding the real economy; it is disputed whether changes in the money market cause changes in real markets. The money market itself consists of a money supply (typically fixed for any given period) and a demand function for money. Because the quantity of money is typically exogenously given, the only variable determined in the money market is the price of money.
The development of money
The earliest discovered forms of money were easily-carried items with some inherent value, such as salt (used in ancient Rome) and fishhooks (American Indians). This replaced or supplemented pre-existing barter systems.
Precious metals, such as copper, gold, and silver, have made good coins, being both easily transportable and having inherent value. In 1933, the United States abandoned the Gold Standard, switching over to Fiat Currency, which has no inherent value beyond the government's assurance that it has value.