Because the value of one currency is only relevant when put in terms of another, FX traders will always deal in currency pairs.

With EURUSD for example, the first currency in the pair (Euro) is the base currency. The second currency in the pair (US Dollar) is the counter currency.

The price of EURUSD tells traders how many units of the counter currency are needed to buy one unit of the base currency. If EURUSD is trading at 1.2500 - then one EURO buys $1.2500 US.