Biodiesel is a liquid fuel for diesel engines, but it is not pumped from oil wells. It is made from vegetable oil or animal fat — used cooking oil, soybean oil, and so on. You can often put it in a diesel bus or truck instead of ordinary diesel, or mix the two. It is a technical and news word, not a kitchen word. Though actually, bio here means “made from plants or animals”, not “harmless” and not “always better”. Growing the crops, cleaning the oil, and burning the fuel still affect the air and the land. It also is not the same thing as petrol (gasoline), and it is not the same as biogas, which is a gas, not a liquid.