Beer is full of tannins.
Whilst the term tannin derives from oak gall or whatever else was used to tan leather, these days the words tannins and polyphenols are essentially interchangeable (one is the other). Hops anyone..lots of polyphenols, malt a suprising amount. It is the tannins that help head retention , they help hot break and cold break by bonding with proteins, they also cause chill haze by the same process.
As the more descriptive name implies there are lots and lots of polyphenols.
The ones we need worry about are the big fat lazy Norm's, not easily dissolved in the mash chaps which co-incidently are found in the husks, closer to the Tele I guess!. As others have pointed out if you do not sparge too long (say 1015 max) or too hot then the pH will not rise to the levels (not sure something like 6) that allows these blokes to dissolve and make it through to you beer. The problem is that they oxidize and cause compounds that tan your hide, well your tongue at any rate. A gross overkill on hops (AKA "Paintstripper") has a similar effect.
Enjoy your Tannins.....(pick up any tree hugging tea packet and read about the benefits of anti-oxidants).
K