A biodiesel car runs on biodiesel which is basically vegetable oil that has been refined an extra step. Nearly all biodiesel cars run on diesel (and sometimes gasoline) as well. You have to buy the fuel at a station that sells it of which there are a few hundred in the U.S. (mostly in the middle west.) In THEORY your car would also run on a vegetable oil from the supermarket but, that's (a) much more expensive and (b) likely to damage your car.