All currencies are identified by the three-letter ISO code. Examples of major currencies: USD (US dollar), AUD (Australian dollar), EUR (euro), JPY (yen) and GPB (British pound).
When trading, foreign currencies are quoted in pairs. In any currency pair, the first currency is called the base currency, and the second currency is called the quote or counter currency. Let's say EUR/AUD is trading at 1.6163. This means that to buy 1 unit of Euro you will need 1.6163 Australian dollars.