Salt is very simple to store, actually. It must be kept dry if you want it easy to use. Getting 3 pound boxes from the warehouse club is fine, just pop them in a sealable plastic bag before putting away. The very best thing is as long as it is dry it stays good as long as the container does.
Salt dissolves in water, but you can evaporate the water back out but that is a lot of energy and work.
There are many different kinds of salt, we have 6 different kinds of salt in our kitchen and there are plenty more out there, but we'll talk about only a few:
Rock Salt or Ice Cream Salt
Table Salt is iodized. The iodine is included to prevent goiter. It is certainly something to have in your stores for that reason. The only downside is it also tastes a little metallic, so you may not want to use it for preserving food.
Kosher salt is just pure salt in bigger crystals, it doesn't have iodine added so if you store this make sure to plant iodine rich foods in your garden or some table salt.
It isn't called Kosher because the salt is Kosher but because it is used in the koshering process. The large crystals are very good at drawing the liquids from meats by just rubbing them on the outside. This also makes it really good for dry curing meats, like country hams.
Different brands have different sizes of crystals that makes thing interesting. I got this conversion table from Cook's Illustrated:
1/4 Cup table salt = 1/2 Cup Diamond Crystal Kosher Salt = 1/4 Cup + 2 Tablespoons Morton Kosher Salt.
Pickling Salt is a very finely ground pure salt. It is specifically made to dissolve easily in cool water. This makes it great for making pickles and brines in general. If you like to cook pork or chicken or turkey this is great to have handy. It also makes good topping for popcorn.
The great thing is if you have Kosher salt you can make your own pickling salt by whirling it up in a food processor or blender. You're just making a finer grind on the salt.
Ice Cream Salt or Rock Salt is also found often in grocery stores. This is not an edible salt, it tends to have high concentrations of minerals making it taste bad and is usually mined. It is used for reducing the freezing temperature of water so you can make ice cream or de-ice your driveway. This is also used for water softeners.
The reason the Fahrenheit scale 0° is where it is, is because that is the coldest you can get with a 1 part salt and 3 part ice mixture.