Cork is Cork Tree bark.
The cork tree is a member of the oak family (Quercus suber).
The bark is removed every 9 years by hand. No tree felling required.
100% natural, renewable, recyclable and biodegradable.
If you want to learn more on Cork check out this site: http://corkforest.org/index.php
Here's an interesting bit of info from their site:
Screw caps are not made from a sustainable product; they are not currently being recycled in the US and are not biodegradable. There is evidence to show that there may be some health concerns from the plastic seal in the screw cap, when closing wine. In comparison to a natural cork, 24 times more greenhouse gasses are released and over and 10 times more energy is used when making one screw cap. Plastic closures are made from petro-chemicals, are not biodegradable and are rarely recycled. They are not sourced from a sustainable product and produce 10 times more greenhouse gasses than natural cork to produce.
Consider this the next time you buy a bottle of wine!