Currency evolved from two basic innovations. Originally money was a form of receipt representing grain stored in temples of ancient civilizations. Then metals were used as symbols to represent value stored in the form of commodities. The world evolved and so did the forms of currency. From Copper to Gold and now to paper, it has seen multiple evolutions.
A currency, according to free dictionary, refers to money in any form when in actual use or circulation as a medium of exchange, especially circulating banknotes and coins. A more general definition is that a currency is a system of money (monetary units) in common use, especially in a nation. Under this definition, US dollars, Indian Rupee, British pounds, Australian dollars, and European euros are examples of currency. These various currencies are recognized stores of value and are traded between nations in foreign exchange markets, which determine the relative values of the different currencies. Currencies in this sense are defined by governments, and each type has limited boundaries of acceptance.