^ The total sum is 200% because each currency trade always involves a currency pair; one currency is sold (e.g. US$) and another bought (€). Therefore each trade is counted twice, once under the sold currency ($) and once under the bought currency (€). The percentages above are the percent of trades involving that currency regardless of whether it is bought or sold, e.g. the U.S. Dollar is bought or sold in 88% of all trades, whereas the Euro is bought or sold 32% of the time.
Forex is a portmanteau of foreign currency and exchange. Foreign exchange is the process of changing one currency into another currency for a variety of reasons, usually for commerce, trading, or tourism. According to a recent triennial report from the Bank for International Settlements (a global bank for national central banks), the average was more than $5.1 trillion in daily forex trading volume.