Chemically, it's a molecule of niacin with a methyl group attached. It breaks down into pyridines, which give coffee its sweet, earthy taste and also prevent the tooth-eating bacterium Streptococcus mutans from attaching to your teeth. Coffee fights the Cavity Creeps.
In this month's issue of Wired (17-10), they give Cromely's World a treat and explain exactly what chemicals you will find in coffee. It turns out to be filled with stuff that's good for you. At least that's I how choose to read it.