Any material from which energy can be obtained is called a fuel.
There are three types of fuel:
1. Bio-fuel: Fuels that are made from the seeds of various plants. Example, sunflower seeds, groundnut seeds, etc.
2. Nuclear fuel: Radioactive elements like uranium 238 decay to form elements of lower atomic numbers. This is called fission. The process releases a lot of energy.
3. Fossil fuels: Coal, petrol, diesel, gasoline, etc are the common fossil fuels that we use in our vehicles or for heating the interiors of homes.
4. Batteries: These store the electrical energy in them. They are used to power toys, modern vehicles, indoor lighting, etc.
5. Biological compounds: ATP, GTP, various sugar molecules, etc.