Crude oil is the "unprocessed" oil that comes out of the ground. It is also known as petroleum. Crude oil is a fossil fuel, meaning that it was made naturally from decaying plants and animals living in ancient seas millions of years ago. Crude oils vary in color, from clear to tar-black, and in viscosity, from watery to almost solid.

Crude oil is classified into various grades according to density (heavy vs light) and sulphur content (sour vs sweet). There are three primary benchmarks of crude: West Texas Intermediate, Brent Blend, and Dubai. West Texas Intermediate is the major crude oil benchmark traded in the U.S. on the New York Mercantile Exchange (NYMEX). It is light and sweet thus making it ideal for producing products like low-sulfur gasoline and low-sulfur diesel.