Though there are many compounds present in the extracts of vanilla, the compound predominantly responsible for the characteristic flavour and smell of vanilla is known as vanillin.
Vanilla essence comes in two forms: the actual extract of the seedpods, and the far cheaper synthetic essence, basically consisting of a solution of synthetic vanillin (4-hydroxy-3-methoxybenzaldehyde):

Natural vanilla is an extremely complicated mixture of several hundred different compounds, versus synthetic vanillin which is derived from phenol and is of high purity. However, it may be difficult to determine the difference between natural and synthetic vanilla flavouring.