A fuel is a substance that is changed in some way, usually by burning, and is one method used to produce heat, electricity, or other forms of energy. Fuels can be divided based on their origin into two main categories: fossil fuels and non-fossil fuels.
Fossil fuels come from that fossils of organisms that have decomposed over a long period of time, such as gasoline, natural gas, and coal. Fossil fuels create carbon dioxide and other greenhouse gasses when burned and take thousands of years to renew and are not sustainable.
Non-fossil fuels do not come from fossils. Some, such as hydrogen gas, do not contain carbon and do not create carbon dioxide or greenhouse gasses. Others, such as wood, ethanol, or biodiesel are a source of pollution. Unlike fossil fuels, non-fossil fuels are renewable and can be sustainably made.