A thick, flammable, yellow-to-black mixture of gaseous, liquid, and solid hydrocarbons that occurs naturally beneath the earth's surface, can be separated into fractions including natural gas, gasoline, naphtha, kerosene, fuel and lubricating oils, paraffin wax, and asphalt and is used as raw material for a wide variety of derivative products. (American Heritage Dictionary)
The word petroleum comes from the Latin petra, meaning “rock,” and oleum, meaning “oil.”
The oil industry classifies "crude" by the location of its origin and by its relative weight or viscosity ("light", "intermediate" or "heavy"). The relative content of sulfur in natural oil deposits also results in referring to oil as "sweet," which means it contains relatively little sulfur, or as "sour," which means it contains substantial amounts of sulfur.