If you read about wine (even here in my little blog), you may often see the word 'tannins'.
Not everybody really knows what it is and if you are like me before wine school, you graciously look over the word...
The days _I_ met the tannins were at school. Like with new friends coming to town: I did not know what to think about them... They invaded my mouth, took over and came to stay. After some 8-12 different wines (we were spitting!, do not worry), they had taken total control over me. The sensations around my gums (the areas holding the teeth) were shifting from a gentle prickling to brutal force. I felt like standing in a wind channel, the whole mouth area being pulled back, like some kind of extreme facial-lifting (nope, did not help, by the way), where my mouth was turned inside and out. The area in the middle of the gums was numb. Dead. And: while I was busy fighting off this total new 'mouth feel', my class mates already started describing the lovely fruits and aromas they detected in their wines. Ha! To me - there was nothing but tannins: Fruit: tannins, Acidity: tannins, Body: tannins, Finish: tannins. (So, really, I should go and buy home some of the first wines we tasted in school to see how the difference would be today!) Luckily, I got over it (took a while, though) and - like with new friends - first got used to them and now I like them (to a certain degree).
However, some people like to have wines with strong and tough tannins, other people want them softer and velvety and some do not like wines with tannins at all. All is possible and a lot (!) is of course depending on the food that is to go with a wine. Certain food will make the tannins of a wine appear less or/and softer than the same wine would taste with another dish (food-wine matching is a whole own topic...). That is why some wines are great to drink as a social-wine and others will get to their best when served with food.
But what is the tannin of a wine? Funny enough the word itself has its roots in the old German language: tanna = tree (you all know the Tannenbaum - Christmas tree, right?). They are natural organic, bitter and astringent compounds found in the skins, the seeds and stems of the grapes. They are further found in the wood of the barrels where some wines will be maturing in. Black grapes generally have more tannins than green ones and red wines are more often left to soak with their skins, sometimes even with the stems on, under a period of time. That is why a red wine is more likely to give the tannin-sensation compared to a white wine. These tannins you will detect with your gums (the parts holding your teeth, or whatever is left of them). The more distinct or/and stronger the sensations are, the more tannins are found in your wine. (It can be many tannins in a wine, but they can still be gentle. And there can be not so many tannins but they can be rough.)
Then there is the part of the wine's maturation in oak barrels, which further adds tannins to a wine. Also that is done to the largest extent with red wines. But there are a good number of 'oaked' whites out there, too. ('Typically' American Chardonnay, i.e.) Tannins from barrel ageing you will mostly feel with the middle part of your gums (the part which has close contact with your tongue...).
Tougher tannins will cause that astringent feeling, that will leave your mouth all dried-out (my wind channel feeling). Softer tannins will add a dimension to a wine, make the complete experience richer (if you like tannins in the first place), but will not take over and... will not hurt. Whether the tannins to you feel rougher or softer is highly subjective.
Tannins are also found in herbs, teas and more. I am sure there is a book 'The world of tannins' out there somewhere...