A commodity is a product which can be either raw material or finished product, it is a  useful thing valued by its function as a physical commodity in the market.
A currency is a form of money in common use in circulation as a medium of exchange. the exchange rate is considered as a currency value.
But when it comes to commodity trading and currency trading or otherwise termed as forex market Commodities trading is done the basis of the underlying value of the asset like gold , silver , crude oil, aluminum, copper, zinc, natural gas etc and forex market deals the currency value of various currencies. It includes the determination of the exchange rate and buying and selling of currencies at a respective prices,