Corks are made from the bark of the cork oak tree, Quercus Suber. They are harvested commercially for the most part in regions surrounding the Mediterranean, most notably in Portugal.
Agglomerated corks are made from cork pieces glued together. Inexpensive and easy to handle, these are suitable for wines that will be held for six months to a year. These are the cheapest corks, and because they tend to be less dense, are the easiest to insert after soaking.
Synthetic corks are made from resins. They are difficult to insert, won’t soften when they are soaked and are difficult to get out without breaking. They are not really suitable for the home winemaker.
Natural corks are punched out directly from the bark in one piece. They are the best quality and will usually be more expensive than the other categories of cork. Better quality cork will allow you to store and age your wine. The cheapest cork will do if you are not thinking of long storage. But, you get what you pay for and a good cork will protect your wine much longer.
If you are intending to age your wine – You should leave your wine bottles standing upright after corking to allow the compressed air above the wine to dissipate. After two or three days, put the bottles on their side for storing. The wine against the cork will keep it moist, keeping the cork plump and so stop leaks. Wipe off the corks as you lay them down to remove any wine that may have been left when you put the corks in.
For more on wine cork production see the Cork Quality Council website