4-hydroxy-3-methoxy benzaldehyde
 H--C--O      H
    |   \    /
    H    C--C      O
        //  \\    //
 H--O--C      C--C
        \    /    \
         C==C      H
        /    \
       H      H

Vanillin, 4-hydroxy-3-methoxy benzaldehyde1, is an aldehyde and the main chemical that gives vanilla its characteristic flavor and scent, and the chief chemical in vanilla extract. These days, much of the supply of vanillin is synthesized from scratch, although natural and almost-natural (derived from naturally-produced chemicals) vanillin is still manufactured. Vanilla extract contains vanillin, but also contains other chemicals that flesh out the vanilla flavor. (After further research I will update this with those compounds).

Update: I have been informed by Professor_Pi that vanillin actually has a methoxy group at the 3 position. My bad. I'll blame my source though. I have made corrections.

1At least that's what I came up with when trying to construct the IUPAC name. /msg me if you think it's incorrect.

Sources: Organic Chemistry, Second Edition by Ralph J. Fessenden and Joan S. Fessenden (my chemistry teacher's college textbook) and myself. Comments welcome, /msg me any time.

July 14, 2005 stealth edit: Thanks to Excalibre for notifying me about the missing hydrogen on the ring.