Vanilla is the pod of a climbing orchid and widely grown in the tropics, especially Madagascar, the West Indies and Fiji. The bean pods are cured and turn black. The extract and essence are both made by soaking vanilla pods in alcohol to leech out the flavor, but vanilla essence, which often has syrup added, is more concentrated. Look for the word 'pure' on the label and avoid artificial vanilla, which is derived from clove oil.
Vanilla pods offer the best flavor of all, for they have subtlety; vanilla essence and extract can be rather overpowering when used with a heavy hand.