Forex trading often involves the simultaneous buying of one currency and selling of another. Therefore, when you buy a currency pair, you buy the base currency and sell the quote currency. The "bid" is the price at which you can sell the base currency at the same time of buying the quote currency. The "ask" is the price at which you can buy the base currency at the same time of selling the quote currency. The bid price is always lower than the ask price. The difference between the bid and the ask price is referred to as the spread.
Source: http://www.investorguide.com/igu-article-1148-forex-basics-currency-pairs.html
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