A currency is a unit of exchange, facilitating the transfer of goods and/or services. It is one form of money, where money is anything that serves as a medium of exchange, a store of value, and a standard of value. A currency is the dominant medium of exchange. To facilitate trade between currency zones, there are exchange rates, which are the prices at which currencies (and the goods and services of individual currency zones) can be exchanged against each other. Currencies can be classified as either floating currencies or fixed currencies based on their exchange rate regime. In common usage, currency sometimes refers to only paper money, as in coins and currency, but this is misleading. Coins and paper money are both forms of currency.

Early currency
The origin of currency is the creation of a circulating medium of exchange based on a unit of account which quickly becomes a store of value. Currency evolved from two basic innovations: the use of counters to assure that shipments arrived with the same goods that were shipped, and later with the use of silver ingots to represent stored value in the form of grain. Both of these developments had occurred by 2000 BC. Originally money was a form of receipting grain stored in temple granaries in ancient Egypt and Mesopotamia.

This first stage of currency, where metals were used to represent stored value, and symbols to represent commodities, formed the basis of trade in the Fertile Crescent for over 1500 years. However, the collapse of the Near Eastern trading system pointed to a flaw: in an era where there was no place that was safe to store value, the value of a circulating medium could only be as sound as the forces that defended that store. Trade could only reach as far as the credibility of that military. By the late Bronze Age, however, a series of international treaties had established safe passage for merchants around the Eastern Mediterranean, spreading from Minoan Crete and Mycenae in the North West to Elam and Bahrein in the South East. Although it is not known what functioned as a currency to facilitate these exchanges, it is thought that ox-hide shaped ingots of copper, produced in Cyprus may have functioned as a currency.

It is thought that the increase in piracy and raiding associated with the Bronze Age collapse, possibly produced by the Peoples of the Sea, brought this trading system to an end. It was only with the recovery of Phoenician trade in the ninth and tenth centuries, that saw a return to prosperity, and the appearance of real coinage, possibly first in Anatolia with Croesus of Lydia and subsequently with the Greeks and Persians.

In Africa many forms of value store have been used including beads, ingots, ivory, various forms of weapons, livestock, the manilla currency, ochre and other earth oxides, and so on. The manilla rings of West Africa were one of the currencies used from the 15th century onwards to buy and sell slaves. African currency is still notable for its variety, and in many places various forms of barter still apply.

Modern currencies
Nowadays, the International Organization for Standardization has introduced a three-letter system of codes (ISO 4217) to define currency (as opposed to simple names or currency signs), in order to remove the confusion that there are dozens of currencies called the dollar and many called the franc. Even the pound is used in nearly a dozen different countries, all, of course, with wildly differing values. In general, the three-letter code uses the ISO 3166-1 country code for the first two letters and the first letter of the name of the currency (D for dollar, for instance) as the third letter.

Privately issued currencies
Several large companies issue points to their customers, to be redeemed for products and services produced by that company. Often, a network of companies will join to share in the offering and redemption of points. While these can hardly be considered stable currency systems, they present many of the same features as “legitimate” currency: they are a store of value, issued in discrete units; they are controlled by a central issuing authority; and they have varying rates of exchange with other forms of currency. For example, frequent flyer miles can be bought using U.S. dollars.

Local currencies
In economics, a local currency is a currency not backed by a national government, and intended to trade only in a small area. Advocates such as Jane Jacobs argue that this enables an economically depressed region to pull itself up, by giving the people living there a medium of exchange that they can use to exchange services and locally-produced goods (In a broader sense, this is the original purpose of all money.) Opponents of this concept argue that local currency creates a barrier which can interfere with economies of scale and comparative advantage, and that in some cases they can serve as a means of tax evasion.