A currency pair is a quotation of two different currencies, where one is quoted against another. Currencies are listed in pairs such as EUR/USD or USD/JPY, where the first currency is known as the base currency and the second as the quote currency.
One of the most commonly traded currency pairs is the EUR/USD pair, which is also the most liquid in the world due to its high volume of trading.
Every currency pair has two prices: a bid price (the highest price a buyer is willing to pay), and an ask price (the lowest price a seller is willing to accept). The difference between the two prices is referred to as the spread.