So, salt is always salt, and except for trace amounts of additives, all the brands on the market are identical with regard to chemical composition, nutritional value, and flavor. The major difference is price, with ordinary table salt usually costing about 30 cents a pound, and some so-called gourmet salts selling for over $15 a pound. Caveat emptor.
Another difference is the size of the salt crystals, ranging from very fine* (like the salt on potato chips) to fine (common table salt) to coarse (most kosher salts and sea salts) and very coarse (rock salt). One major salt producer has over sixty grades of salt for different commercial purposes. So how do all these different grades of coarseness effect us?
Well, you see, as the grains of salt get bigger, so does the amount of space between the individual grains. In other words, a tablespoon of finely powdered salt will contain more salt by weight than a tablespoon of coarse salt because there is less air space between the small grains than the large grains. Make sense? Furthermore, the smaller the grains of salt, the faster they dissolve in a liquid - and in our mouths.
This can lead to some misleading sensory impressions. People frequently think that a certain salt tastes more or less salty than another when the coarseness of the grain is entirely responsible - not the nature of the salt itself, but rather the size of its crystals. You can easily prove this to yourself by popping a single large grain of salt into your mouth, followed by another large grain of salt which you have pulverized with the back of a spoon. The second grain of salt will dissolve on your tongue faster, giving you a faster and stronger taste of salt even though the two samples are identical in every way except for the size of the crystals.
This same factor also effects measurements. Since a tablespoon (or cup, or gallon) of coarse salt will actually contain less salt by weight than an equal volume of fine-grain salt (remember those air spaces between the grains?), precise measurements by volume cannot be relied on. Notice I almost always say "salt to taste" in my recipes? That's why. This has also led some people, including cookbook authors, food writers, professional chefs, and Madison Avenue copy writers, all of whom should know better, to pronounce that one salt is "less salty" or "more salty" than another.
One more time - salt is salt - got it?