What is biodiesel?

A mix of modified vegetable oils (that's the "bio" part) and conventional diesel fuel. There's a small but growing market for biofuels, which supply about 1 percent of the world's transportation fuel.
How is biodiesel sold?
Biodiesel is usually sold in ratios of 10- or 20-percent modified vegetable oil and 80- or 90-percent conventional diesel. These fuels are called B10 or B20. The B factor tells you the percentage of modified vegetable oil. In other words, B100 is pure modified vegetable oil, while B20 is 20-percent modified vegetable oil and 80-percent petrodiesel.
Soybean oil is usually the raw material in the vegetable part of the fuel, but rapeseed oil and animal fats can be used, too. Experiments with Vitalis, Cheese Whiz and Soy Pez remain inconclusive.