Petrol is a fuel derived from petroleum. If you are American, you probably call petrol "gas."
When the price of petrol goes up, lots of people complain, because petrol is gasoline. Petrol is the fuel used in many types of vehicles, including cars, motorcycles, motorboats, and airplanes. Petrol is derived from petroleum, and besides smelling bad, it's very flammable. If you light a match near petrol, there will be an explosion. Since petrol comes from petroleum, petrol is a fossil fuel.