A fossil fuel is any hydrocarbon that may be used as a source of energy. Coal, oil and natural gas are the three basic fossil fuels, formed 300 million years ago from organic plant and animal life subjected to great heat and pressure under the ground.They are classes as non-renewable sources of energy.ossil fuels were formed from prehistoric plants and animals that lived millions of years ago. When these living things died, they decomposed and got buried under the layers of mud, rock and sand. Millions of years passed and the dead plants and animals slowly decomposed into organic material and fossil fuels.