The United States Mint was created by Congress under the Coinage Act of 1792 and became part of the Department of Treasury in 1873. The Mint manufactures all legal tender for the United States, including the US Dollar (USD) and coins for trade and to collect uncirculated. The Act gave a specific value of silver to a dollar and gold to an eagle. Further, it made the value of an eagle at 10 dollars and the dollar at 1/10 of an eagle. It made 90% silver alloy coins in denominations of 1, ½ ¼ and 1/10 eagles. The US Dollar is no longer backed by gold, silver, or other precious metals.
USD is considered a fiat money and is now printed as legal tender on a special paper for trade. The value is derived from government regulation, supply & demand, trade sentiment on the foreign exchange market, and financial stability based on employment situation, inflation/deflation, and imports/exports.
Some laws that govern US legal tender include the Coinage Act of 1965, USA PATRIOT Act, and the most recent Dodd Frank Act.
The US dollar is the most used currency in the world. A currency pair cannot be considered a major pair without comparing to the US dollar (USD).