Money is a medium of exchange. It allows two people to trade without needing what the other wants. This can come in all types and forms. So long as people trust and accept a certain form of money; it can be used as a medium of exchange. For example, gold was historically used as a form of monies. Farmers would go to market and sell their cows in exchange for a certain amount of gold. This gold could then be exchanged by the farmer for a loaf of bread from the baker.
Money supply looks at what money is and how it is controlled.