Currency that is backed by a tangible asset like a precious metal.
The Founding Fathers of the United States overwhelmingly favored specie (gold and silver coins) as the basis of our currency. Historically the stablest long term basis of monetary policy has been hard money.
Currency that is backed by nothing but the promises of government.
In the United States legal tender laws are used in conjunction with a coercive government monopoly to give the privately owned Federal Reserve System total control of the monetary policy of the United States.