A summary treatment of crude oil follows. For full treatment, see petroleum, petroleum production, and petroleum refining. Crude oil is a mixture of comparatively volatile liquid hydrocarbons (compounds composed mainly of hydrogen and carbon ), though it also contains some nitrogen, sulfur, and oxygen. Crude oil and other liquids produced from fossil fuels are refined into petroleum products that people use for many different purposes. Biofuels, such as ethanol and biodiesel, are also used as petroleum products, mainly in mixtures with gasoline and diesel fuel. Petroleum is the largest U.S. energy source. We use petroleum products to propel vehicles, to heat buildings, and to produce electricity. With that said, the four main types of Crude Oil are: Very light oils – these include: Jet Fuel, Gasoline, Kerosene, Petroleum Ether, Petroleum Spirit, and Petroleum Naphtha. Light oils – These include Grade 1 and Grade 2 Fuel Oils, Diesel Fuel Oils as well as Most Domestic Fuel Oils. Medium oils – These are the most common types of Crude Oil.