You can search this site for the term "fiat money" and find many answers there. Basically, the silver and gold standards guarantee that there is a valuableable trade item to back up the value of currency. Paper currency (fiat money) is issued by governments and dependent upon the governed body in their governing power. Fiat money allows for greater trade latitude (import/export), whereas gold and silver based currency relies not on government confidence, but on actual possession of resource.
It is interesting to note that throughout history, all major economic systems in major societies depending upon fiat money have failed.
It is interesting to note that throughout history, all major economic systems in major societies depending upon fiat money have failed.