Cork is the outer bark of a variety of oak tree. It is stripped off without damage to the tree. There can be shortages in supply though.
There is a shortage of the quality of bark that you can drill full size corks out of in the traditional way. Most cork products are made from ground up bark.
The corks in the Crane Lake wine which is my daily guzzle have 1/8 inch layer of cut cork at each end. These disks are bonded to the body which is made of bonded ground cork. These are very good corks with no seepage problems at all, and are much nicer than foam plastic corks.