The first currency in a currency pair (the one on the left side from the slash "/ ") is called base currency. Base currency always has a value of 1. The second currency in the currency pair (the one on the right side from the slash "/") is called counter currency or quote currency. For example: EUR/USD  

·         EUR – base currency with value of 1.

·         USD – quote (or counter) currency

Think of the base currency as the “root”. When you buy, for example EUR/USD, it means that you are buying the base currency (EUR) and simultaneously selling the quote currency (USD).