A commodity is some good for which there is demand, but which is supplied without qualitative differentiation across a market. It is fungible, i.e. the same no matter who produces it. Examples are petroleum, notebook paper, milk or copper. The price of copper is universal, and fluctuates daily based on global supply and demand. Stereo systems, on the other hand, have many aspects of product differentiation, such as the brand, the user interface, the perceived quality etc. And, the more valuable a stereo is perceived to be, the more it will cost.

In contrast, one of the characteristics of a commodity good is that its price is determined as a function of its market as a whole. Well-established physical commodities have actively traded spot and derivative markets. Generally, these are basic resources and agricultural products such as iron ore, crude oil, coal, ethanol, salt, sugar, coffee beans, soybeans, aluminum, copper, rice, wheat, gold, silver and platinum. Soft commodities are goods that are grown, while hard commodities are the ones that are extracted through mining.