Most great wine is aged in wooden barrels, and these barrels have to come from somewhere. A cooper is the person that actually constructs these barrels. In order to become a cooper, you need to be a craftsman with some carpentry skills, an eye for detail and in-depth knowledge about wood.
French oak is the most popular kind of wood and also the most expensive to use. Wine barrels are made from strips of this wood and joined tightly together before being bound by metal hoops, with a single, corked hole in the center of the barrel. They're usually made in Europe, although there are cooperages in California as well.