A fuel is any material that can react with oxygen to release energy from a potential form into a usable form. There are many different types of fuel.

Solid fuels include coal, wood and peat. All these types of fuel are combustable, they create fire and heat. Coal is a fossil fuel extracted from the ground by mining. It is a readily combustible black or brownish-black rock. It is composed primarily of carbon and hydrocarbons, along with assorted other elements, including sulfur.

Non-solid fuels include oil and gas (both fuel types have various varieties).

Oil is a generic term for fluids that are not miscible with water. The name comes from Latin oleum for olive oil. In the United States, petroleum is referred to predominantly as oil. For example, an "oil shortage" would mean an inadequate supply of petroleum rather than cooking or mineral oil. Oil is used frequently in politics and the media when referring to dependence on "foreign oil", or oil that is imported from other countries.

Crude oil consists of a mixture of petroleum liquids and gases (together with associated impurities) pumped out of the ground through oil wells.

Petroleum (from Latin petrus–rock and oleum–oil) or mineral oil is a thick, dark brown or greenish flammable liquid, which, at certain points, exists in the upper strata of Earth's crust. It consists of a complex mixture of various hydrocarbons, largely of the methane series, but may vary much in appearance, composition, and properties. It can be shortened to the prefix petro-, as in "petrodiesel".

Natural gas, which is about 80% methane, with varying proportions of ethane, propane and butane, and is used as a fuel.

A fuel is not necessarily combustible. For example, in a nuclear reaction a fuel will undergo fission. This still provides a useful source of energy but not via combustion. Also, in stars (and our sun), hydrogen is the fuel for the nuclear fusion.

In the bodies of most animals, the fuel sources are carbohydrate, fat, protein, which supplies the energy for muscles.