So what exactly is a polyphenol? A polyphenol is defined as a compound that possesses a phenolic moiety. Okay, so what's a phenolic moiety? For our purposes, we'll define a moiety as a functional group - the atoms within the molecules that are responsible for the characteristic chemical reactions of the molecule (this isn't exactly what a moiety is, but it works for now). So the OH part of an alcohol would be the functional group that defines it as an alcohol.
A phenolic is a functional group. It has a benzene ring (a ring of carbon that has 3 double bonds within it) and a hydroxyl group (the -OH). (The actual definition is a class of chemical compounds consisting of a hydroxyl group (OH) bonded directly to an aromatic hydrocarbon group - the benzene ring part.) Although they have the -OH of the alcohol group, they don't behave like alcohols because they are attached to the ring, not a nice straight line of carbon atoms. They have higher pH levels than alcohol - 10 to 12 - and they can be called a carbolic acid.
Phenols can have extra functional groups or can connect with other phenols to create all kinds of interesting polyphenols (the word "poly-" means many). This is hydroquinone - you can see a hydroxyl group has been added to the ring (at the bottom) to create a new compound.
So why do we care? Because polyphenols offer all kinds of amazing benefits to cosmetic products! The major categories are flavonoids, catechins, and lignans, each of which brings something great for our skin! (I'll be mainly focusing on the flavonoids over the next few days!) Flavonoids are classified by their biosynthetic origin - those that are intermediates and end products of biosynthesis, and those that are only end products.
Flavonoids behave as anti-oxidants on our skin and in our bodies by scavenging the free radicals produced at our cell membranes. It is thought (meaning there aren't enough studies or nothing conclusive) the flavonoids offer anti-inflammatory benefits by inhibiting pro-inflammation mediators in our bodies, such as prostaglandins. Some flavonoids have anti-biotic, anti-fungal, and anti-reddening qualities.
For the purposes of our discussions on extracts, I'm going to call them all polyphenols, as it's simply easier that breaking it down into flavones, flavonols, flavanols, and so on.