The dollar sign, also known as peso sign, is a currency symbol consisting of a capital ⟨S⟩ crossed with one or two vertical strokes, used to indicate the unit of various currencies around the world, including most currencies denominated "peso" and "dollar". The explicitly double-barred sign is called cifrão in the Portuguese language.
A piece of eight from the Potosí mint, showing the Pillars of Hercules with "S" ribbons, and two "PTSI" monograms at about 4 and 8 o'clock around the edge
Car for sale in Cape Verde, showing use of the cifrão as decimals separator
A currency is a standardization of money in any form, in use or circulation as a medium of exchange, for example banknotes and coins. A more general definition is that a currency is a system of money in common use within a specific environment over time, especially for people in a nation state. Under this definition, the British Pound sterling (£), euros (€), Japanese yen (¥), and U.S. dollars (US$) are examples of (government-issued) fiat currencies. Currencies may act as stores of value and be traded between nations in foreign exchange markets, which determine the relative values of the different currencies. Currencies in this sense are either chosen by users or decreed by governments, and each type has limited boundaries of acceptance; i.e., legal tender laws may require a particular unit of account for payments to government agencies.
Cowry shells being used as money by an Arab trader
The world's oldest coin, created in the ancient Kingdom of Lydia
Song dynasty Jiaozi, the world's earliest paper money
A list of exchange rates for various base currencies given by a money changer in Thailand, with the Thailand Baht as the counter (or quote) currency. Note the Korean currency code should be KRW.