Cork oaks are harvested every nine years, once they reach maturity. It doesn't harm the tree, and the cork bark regrows. Most cork forests are in Portugal and Spain. The year of harvest is marked on the trunk, so each tree isn't harvested at the wrong time.
Why wine bottles typically use cork ?
Cork is made from bark, which makes it a renewable resource. Plus, it can form to the shape of a wine bottle, making it an incredibly appealing way to seal wine. But there's a drawback: Occasionally bad cork can get into the wine, something called "cork taint".