Cork is a lightweight material made from the bark of a particular kind of oak tree. Cork is used in all sorts of ways, from flooring to fishing rod handles to bulletin boards.
When cork bark is harvested from a cork oak tree, it doesn't harm the tree itself — this quality is part of what makes cork a useful, renewable material. It shows up in shoes, insulation, wall tiles, and badminton shuttlecocks, as well as bottle stoppers, which are also called corks. The word is from the Spanish alcorque, "cork-soled sandal," which ultimately comes from the Latin name for the cork oak tree, quercus.