What most gourmands would define as "real" and "natural" vanilla flavoring is simple: Vanilla beans steeped in alcohol. But vanillin, the chemical responsible for vanilla's taste and flavor, is a far more complicated beast. Chemically identical to real vanilla, artificial vanilla can be made from clove oil, pine bark, coal tar, bran, even cow dung. Until fairly recently, the chemical lignin, derived from wood pulp, was the most common way of synthesizing vanillin. Most artificial vanilla is now derived from guaiacol, a chemical derived from creosote or Guaiacum flowers.
The United States Food and Drug Administration has thrown a hint of confusion (and a note of lychee) into the cut-and-dry definitions of "real" and "fake" vanilla. Any flavor derived from edible sources can be labeled a natural flavor. Therefore, vanillin made from bacterial fermentation of corn or rice bran is a "natural" vanilla flavor – just not "real" vanilla flavor. However, vanillin made from cow dung, while natural in all senses, is not legally "natural", because dung normally isn't a source of food.
Via Edible Geography.