If you are referring to edible salts (used for cooking) there are all sorts of different kinds.
In the U.S. plain old table salt usually has iodine added to it, which was added back in the early 1900’s to help prevent people from getting goiters. Then there is sea salt, which is pretty self explanatory.
I’ve taken the liberty to post some info from Melinda Lee (my favorite radio cooking host) about Kosher salt. This particular excerpt was from an article about brining turkey:
Kosher salt is the ONLY type of salt to be used in making brine (it is sweeter and more pure than ordinary table salt). Kosher salt is available in two varieties. The most common is flaked salt (example: Morton Kosher Salt) which has been pressed into flakes by rollers. The other type is a four-sided crystal (example: Diamond Crystal Kosher Salt). The crystal-shaped salt measures differently because of its shape. Use about 25% (one quarter) MORE crystal salt than flaked salt when measuring for brine or other recipes. The formula is: 1 tablespoon of regular table salt is equivalent to 1½ tablespoons flaked kosher salt, or 2 tablespoons, diamond crystal kosher salt.
Then there are all sorts of salts that come from different parts of the globe that may be slightly colored by the minerals that are scooped up along with the salt, giving them slight variations of flavor. You can check out This Site to read more about the different types of edible salts and where to use them.
I don’t know anything about the metal aspect of salts though, just that some are edible and some are not and some types of salt work/taste better than others for particular dishes. Happy cooking!