Fossil fuels include coal, oil, natural gas, petroleum, coke or other petroleum-based fuels. They're called fossil fuels because they are formed from the decayed remains of prehistoric plants and animals. Fossil fuels are non-renewable energy sources which have a finite supply. All fossil fuels contain carbon, and when they are combusted to make electricity or power an airplane or automobile engine, they create carbon dioxide (CO₂), a major greenhouse gas.