A fuel is any material that can react to release energy from a potential form into a useable form. In most cases this is just something that will burn, for example, coal was burnt by steam trains to heat water to provide power, or natural gas is burnt to provide heat to run power plants. However a fuel is not necessarily combustable, for example, in a nuclear reaction a fuel will under go fission. This still provides a useable source of energy but not via combustion.
In the bodies of most animals, the fuel used is a carbohydrate, which supplies the energy for muscles.