What is biodiesel?

Very often, a broad, general description is used to define biodiesel in a way that is easy to understand by the general public. However, when these broad descriptions are adopted by an authoritative body as a formal definition, they can include a wide range of experimental fuels that are not biodiesel. The term “biodiesel” has a specific, technical definition that has been agreed to through a painstaking process by members of industry and government which has received full approval by the American Society of Testing and Materials (ASTM), the premier standard setting organization for fuels and fuel additives. That definition is used for purposes such as alternative fuel designation, EPA registration, or other regulatory purposes. Nonetheless, this specific technical definition can be confusing to the general public. We have, therefore, chosen to adopt two definitions for biodiesel. The “general definition” is a simple description for the general public. The “technical definition” should be adopted for use by customers for bid specification purposes or government entities for regulatory purposes.
General Definition of Biodiesel:
Biodiesel is a domestic, renewable fuel for diesel engines derived from natural oils like soybean oil, and which meets the specifications of ASTM D 6751.

Clarifying language to general definition:
Biodiesel can be used in any concentration with petroleum based diesel fuel in existing diesel engines with little or no modification. Biodiesel is not the same thing as raw vegetable oil. It is produced by a chemical process which removes the glycerin from the oil.

Technical Definition for Biodiesel (ASTM D 6751) and Biodiesel Blend:
Biodiesel, n—a fuel comprised of mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats, designated B100, and meeting the requirements of ASTM D 6751.

Biodiesel Blend, n—a blend of biodiesel fuel meeting ASTM D 6751 with petroleum-based diesel fuel, designated BXX, where XX represents the volume percentage of biodiesel fuel in the blend.

Clarifying language to technical definition:
Biodiesel, as defined in D 6751, is registered with the US EPA as a fuel and a fuel additive under Section 211(b) of the Clean Air Act.

Biodiesel is typically produced by a reaction of a vegetable oil or animal fat with an alcohol such as methanol or ethanol in the presence of a catalyst to yield mono-alkyl esters and glycerin, which is removed.

These informations taken from : http://www.biodiesel.org/resources/definitions/