So, I took myself off to that fount of all wisdom and knowledge and benevolence, Wikipedia. It has this to say:
A tannin ... is an astringent, bitter plant polyphenolic compound that binds to and precipitates proteins and various other organic compounds including amino acids and alkaloids.
The term tannin (from tanna, an Old High German word for oak or fir tree, as in Tannenbaum) refers to the use of wood tannins from oak in tanning animal hides into leather; hence the words "tan" and "tanning" for the treatment of leather. However, the term "tannin" by extension is widely applied to any large polyphenolic compound containing sufficient hydroxyls and other suitable groups (such as carboxyls) to form strong complexes with various macromolecules.*
Righto! So that clears that up then! Now that I know that, I just need to find out what "polyphenolic compounds", "proteins", "amino acids", "hydroxyls", "carboxyls" and "strong complexes with various macromolecules" are. Actually, I didn't have time to look all that up, but I did look up what seemed to me to be the most important terms: "polyphenolic"...
Polyphenols.... a structural class of... organic chemicals.... of large multiples of phenol structural units.
Well wank you very much, Dikypedia. So a phenol is....
Phenol, also known as carbolic acid, is an aromatic organic compound with the molecular formula C6H5OH.
A carboxyl.... well, we get redirected here:
A carboxylic acid // is an organic compound that contains a carboxyl group (CO2H).
A hydroxyl is a chemical functional group containing an oxygen atom connected by a covalent bond to a hydrogen atom, a pairing that can be simply understood as a substructure of the water molecule.
Er.... and I'm not even drunk.
And as for "strong complexes with various macromolecules", I'm not even going to go there, as I'm a happily married man.
So there you go folks. That's what tannins are. So know that I know what they are, I still don't know what they are.
*Fun party trick: try reciting all of that while drunk.