Fossil fuels are made up of decayed plant, dinosaur and other animal matter. When the plants, dinosaurs and other ancient creatures died, they decomposed. Eventually their remains became buried deep under the ground. Over millions of years, these plant and animal layers were pressed and squeezed with the earth's heat, changing them into fossil fuels.
Oil is one type of fossil fuel.
A thick, gooey, black liquid, oil is also called petroleum. Natural gas is another kind of fossil fuel. Normally you can't see or smell natural gas. Coal is also a fossil fuel. Coal comes in many forms ranging from hard, black colored rocks to soft brown dirt.