When we use energy we often convert it or change it from one form to another. The energy in fossil fuels or other sources of energy is not always in the form we need. The purpose of energy conversion is to change energy resources into more usable forms.
Fuel is matter-containing energy stored in chemical form.
Food, wood, and coal are all examples of fuels. Energy is packed into the chemical structure of the fuel, and we release it when we convert or change that chemical structure. For example, food contains Calories, units of energy content. We eat food, and our bodies convert it, releasing energy in the forms of motion and heat. It is actually the energy from our food that allows us to move, and that which keeps our bodies at their normal temperature.