Salt that occurs in hard massive layers beneath the ground is called rock salt. These deposits were formed by the evaporation of large parts of oceans millions of years ago. Kosher salt is manufactured according to the Jewish dietary laws and the large irregular grains are ideal for curing meat, which involves absorbing blood from raw meat. Rock salt is used primarily to de-ice driveways and make ice cream. Butter salt has extremely fine grains, and is used to salt butter.
Popcorn salt has very fine grains, which adhere better to popcorn, potato chips, and French fries.
Saltpeter is a preservative, and has been used in the making of brines and dry salt mixtures for hundreds of years. It has been found to be harmful when eaten in large quatntities, so it is probably wise to think twice before using it. Adding salt to water will raise the temperature at which it boils and lower the temperature at which it freezes. Adding a little salt balances the flavor of sweets like cakes, cookies, and candies. Boiling eggs in salted water makes them easier to peel.