Fiat money is what is regular in modern economic systems.  Fiat money is money that is described as legal tender by either a government or some organization with the authority to explain legal tender.  In the United States the Federal Reserve System issues Federal Reserve Notes, which give as the legal tender for the United States.  The currency used here is backed by nothing excluding the faith of the general public that this money will be acceptable by everybody else with whom you could have an economic transaction.