This comes from Richard White, the Copacetic Cook, a chef in Germany. I asked him your question since I've been curious about the same thing and had gotten no real answers.
Sea salt is exactly that.
It is salt that has been harvested out of salt marshes, bays or special enclousure after the sea water has evaporated. I always bring back a kilo or two when I'm in France. It is my personal preference, I use it exclusively. Hope this is of some help. Alert us of bad posts.