What is biodiesel, anyway?
Biodiesel, the common name for fatty acid alkyl esters, is a biological-based fuel that can substitute petroleum fuels in diesel engines. It can be made from virgin vegetable oil, animal fats, algae, or from recycled oils like those used by restaurants for deep-frying. Biodiesel burns cleaner than fossil fuel diesel and has less carbon monoxide, aromatic hydrocarbons, and particulate matter (soot). It does, however, release more nitrous oxide than regular diesel, a factor in smog. Biodiesel is often called a carbon neutral fuel because while it does release carbon dioxide, it is the same carbon absorbed by the plant (or animal) source from which the fuel came. Fossil fuels, on the other hand, release carbon dioxide that could otherwise remain sequestered below the earth's surface, out of the atmosphere.
And what's veggie oil?
Diesel engines can also run on straight vegetable oil (SVO), but modifications are most often required. Since vegetable oil has a higher viscosity (is thicker), it needs to be heated before it can flow properly. A veggie oil conversion is a system that, in one of several ways, heats the vegetable oil to the appropriate temperature before burning it in the engine.
Read about the creepy idea of running cars on human fat