The U.S. Dollar (USD) is the currency of the United States. The U.S. dollar is the most traded currency on the Forex and is present in 85% of total transactions in 2010. In 2001, 90% of transactions were linked to the Dollar. The U.S. dollar is a currency of reference both in the foreign exchange market and in international trade.
The dollar is still considered a safe haven even if the contestations are becoming more numerous. Indeed, the debt of the United States but is becoming alarming, but does not challenge the supremacy of the dollar.
The dollar is also the most used currency in reserve in the world especially in China which has 2.5 trillion dollars in foreign currency reserves.
In terms of banknotes in circulation, the dollar is the second currency in the world. You can see below a $1 banknote :