Manufacturers harvest sea salt by creating shallow ponds that get lots of sun and wind, resulting in the evaporation of the water and concentration of the salt. This salt is formed into mounds and processed for purification. Sea salt is an all-natural product that is part of the earth and doesn't expire. Sea salt with additives can spoil, though.
Sea salt differs in origin from regular table salt, which comes from mineral deposits in the earth. It's said to have a higher mineral content and be healthier, but scientific support of this is inconclusive. Sea salt is primarily sodium chloride.
Sea salt is best stored in the container in which you purchased it. Place in a cook, dark place, such as the pantry, away from moisture and high humidity. While not required, keep it away from temperature extremes too -- such as would be found around the stove or the dishwasher.
Aged sea salt isn't dangerous to eat, it simply is harder to use. You might find it becomes crusty or unpleasantly moist if stored improperly. Some brands of sea salt contain added iodine, which could shorten its shelf life to about five years.