Tannin and Aging Potential
What is Tannin?
Tannin is a substance that comes from the seeds, stems and skins of grapes. (For a taste of heavy-duty tannin, try a strong cup of tea.) Additional tannin can come from the wood during barrel aging in the winery. It is an acidic preservative and is important to the long term maturing of wine.
Through time, tannin (which has a bitter flavor) will precipitate out of the wine, becoming sediment in the bottle, and the complexity of the wine's flavor from fruit, acid and all the myriad other substances that make up the wine's character will come into greater balance.
Generally, it is red wines that are the ones that can produced with a fair amount of tannin with an eye towards long term storing and maturation. However, the drawback is that you shouldn't drink it too young since it will taste too harsh. The good news is that (with a little luck) after a number of years, what you get is a prized, complex and balanced wine.
General Rule for Aging Potential:
Two wineries, side by side, producing the same grapes and the same wine. One ages considerably longer than the other, why?
Whilst they are the same grapes, perhaps the soil or microclimate (small variations in the local weather due to terrain; what the French call 'terroir') is just a bit different. Maybe the vines are older. The winery may have processed the wines differently (for example, heavy filtering). In fact, even the size of the bottle matters--a 1/2 bottle ages faster than larger bottles. There are lots of reasons, so general rules are just that--general.
In any event, the red French Beaujolais Nouveau is meant to be drunk within days. It is a light, fruity wine.
White wine is the next least aged wine. There is a range from a light wine like Sauvignon Blanc or a light Chardonnay, to more 'ageable' complex Chardonnay of good White Burgundies. Probably drink the former within a few years (aging isn't needed, and the latter from 3 to 7 years). Dessert wines like Sauternes, Hungary Tokaji or other late harvest wines (Riesling, Gewurztraminer, etc.) should be aged. Sauternes get better over a VERY long time: 10, 20, 30, 40 or more years!
Then come the reds. While the vast majority of wines produced today CAN be drunk immediately, a good number of red wines will benefit by some aging and some will benefit from a lot of aging. The ones that you open now that taste like road tar may very well be fantastic in 5 or 10 or 20 years. Look to some French Bordeaux (maybe up to 50 years) or Italian Barolo.