Vanilla is a flavour derived from orchids of the genus Vanilla and is second in cost only to saffron. The various subspecies are V. planifolia, grown on Madagascar, Réunion, other tropical areas along the Indian Ocean and Mesoamerica where it originated; V. tahitensis, grown in the French Polynesia; and V. pompona, found in the West Indies, and Central and South America.
Seventy five percent of the world crop comes from v. planifolia and is generally known as Bourbon (the former name of Réunion) or Madagascar. Smaller, and subsequently harder to get, amounts come from Mexico, Polynesia, and the West Indies and are prized by afficionados of the elixir. The intensity of the flavour can vary dramatically.
Cook's Illustrated in a recent and thorough test found that high-quality artificial vanilla flavouring is best for cookies, while high-quality real vanilla is very slightly better for cakes and significantly better for unheated or lightly heated foods.