The U.S. dollar ($ or USD) is the official currency of the United States but also of its many overseas territories.
In terms of transaction volume, the dollar is today (and since a long time) the most used currency, well ahead of the euro. On Forex, about 85% of transactions includes the U.S. dollar.
It is also reference currency in world trade. However, in terms of amount of outstanding notes, the dollar was surpassed by the euro.
The U.S. dollar has always been considered a safe currency. This currency has shown for many crises strength and is one of the reasons why investors to give to dollar their trust.