Crude oil is a hydrocarbon mixture existing as a liquid in natural underground pools or reservoirs. It is a raw material refined or processed into gasoline, heating oil, jet fuel, diesel fuel, propane and chemicals. The higher the specific hydrocarbon content, the "heavier" the "grade" of crude. Different "grades" are sold on commodities markets throughout the world at varying prices.
Currently in the U.S. alone there are about 18 grades of oil; worldwide there are at least 90 grades of crude oil. The benchmark grades are the "light sweet" crude oil or "West Texas Intermediate" crude for the U.S. and North Sea Brent crude for the international market.