Foreing currency is any currency that is legal tender other than the currency used in the country. The term currency refers not so much to the physical currency (paper and metal), but to the nominative term for the currency of another country.
Currencies are quoted on the foreign exchange markets, where their price fluctuates and where they are bought and sold. Each currency has a specific value relative to another in the foreign exchange market and this relationship is called the exchange rate.
The relationship between two currencies can be direct or indirect. An indirect relationship between two currencies means that it is derived from the exchange rate between them and a third currency.
You can see the exchange rate of the currencies in currency quotation and evolution of the currencies.