Fuel that can be used to produce heat. A material such as wood, coal, gas, or oil burned to produce heat. Fuel releases energy either through a chemical reaction, such as combustion or nuclear fission or nuclear fusion.

Fuels used in the industry, transportation and the home are burned in air. The carbon and hydrogen in fuel rapidly combine with oxygen in the air in an exothermal reaction one that liberates heat. Fuels used by industrialized nations are in the form of incompletely oxidized and decayed animal and vegetable materials or fossil fuels, specifically coal, peat, lignite, petroleum and natural gas.
These natural fuels other artificial ones can be derived. Coal gas, coke, water gas and producer gas can be made using coal as the principal ingredient. Gasoline, kerosene and fuel oil are made from petroleum. Fuel must be in a liquid form.

Fossil fuels are hydrocarbons(coal and petroleum) formed from the dead plants and animals by the exposure to heat and pressure in the Earths crust over hundreds of millions of years. Fossil fuel available is limited and new methods of recovery are being developed. Biofuel can be solid, liquid or gas fuel derived from biomass. Biomass can also be used directly for heating known as the biomass fuel. Now fuel is hydrogen, which is employed as a fuel only for a few special purposes because of its high cost. Hydrogen can be produced by electrolysis of water for which nonfossil fuels would supply the energy.

Solar energy is the radiant light and heat from the Sun. Solar energy could be utilized either by direct conversion to electricity using photovoltaic cells. Fuels are rated according to the amount of heat they can produce. Nuclear fuels are also possible substitutes for fossil fuels. Nuclear fuel used by humans is heavy fissile elements that can be made to undergo nuclear fission chain reactions in a nuclear fission reactor. Nuclear fuels are not burned, they undergo reactions in which the nuclei of their atoms either split undergo fission.

Alternative fuels is usually used to mean fuels for motor vehicles that are not gasoline. Alternative fuels also known as non-conventional fuels. Alternative fuels can also assign to any fuel that is not a fossil fuel. Sometimes the phrase is used inaccurately to assign to alternative sources of energy or power.

Biodiesel fuel made from the natural, renewable sources, such as new and used vegetable oils and animal fats, chemically-reacting lipids and alcohol for use in a diesel engine. Biodiesel properties very similar to the petroleum-derived diesel fuel, but its emission properties are better.