What is a fuel?

According to the law of conservation of energy: Energy can neither be created nor be destroyed; it can only be changed from one form to another. Thus, we cannot produce energy to do certain work. Therefore, we use certain substances which help us to transform one form of energy into another form.

For example, when we burn paper with a matchstick, light is produced by the flame. If we analyze this closely, light energy is not created over there, it has just been produced due to the transformation of heat energy provided by the matchstick into light energy. Thus, we always need a certain substance to convert one form of energy into another for accomplishing various jobs. We call such materials as fuels. In other words, any substance that upon combustion produces a usable amount of energy is known as fuel. Example: fossil fuels, biogas, nuclear energy,  etc.