Fuel
A fuel is any material that can react with oxygen to release energy from a potential form into a usable form. In most cases this is just something that will burn.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 was burnt by steam trains to heat water to provide power. Peat and wood are mainly used for domestic and industrial heating, though peat has been used for power generation, and wood-burning steam locomotives were common in times past.
Non-solid fuels include oil and gas (both fuel types have various varieties). The former is widely used in the internal combustion engine while both are used in power generation. However 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.