Anything that is easily used as a medium of exchange in settling a trade is called money.  Earlier demanded commodities like salt, rice wheat, weapons, livestock was used as money, it was called barter exchange. After that metallic money like gold, silver, copper come into existence. They were quiet popular. Soon this money was found inconvenient as well as dangerous to carry, so replaced by paper currency. Every country prints its own fiat currency as per their policy; generally these currencies are printed and controlled by the central bank of that country. After economic revolution paper money is accompanies by credit money, plastic money etc. The latest form of Digital money (bitcoin) gets introduced in the year 2009. Purpose of all forms of money is to make the trade simple and easy.