Salt is mostly sodium chloride. It from two main sources - sea water and rock salt (found in the earth in crystalline form). It is essential to animal life. Herbivorous animal have a greater need for salt than Carnivores. The UK Food Standards Agency recommends that adults consume no more than 6gms (a level teaspoon) a day. Over-consumption may contribute to high blood pressure and other health problems. The same agency says that 75% of our salt intake is from processed foods
Salt is a natural preservative and has been used for centuries. Even with modern refrigeration salt is used to preserve and flavourn meat (e.g. bacon and corned beef), fish (e.g. baccala and anchovies) and vegetables (e.g. capers).
Salt is one of the four basic flavours (sour, sweet, bitter and salty).
Table salt is refined to provide a pure white salt that, with additives, pours easily. It often has iodine added to it to replace that lost in the modern diet.
Cooking salt (Kosher salt in the US) is less refined than table salt and is evaporated from salt water. It contains no additives.
Sea salt, as the name suggests, comes from evaporated sea water. It comes as both crystals and flakes. It includes a number minerals, depending on its source, which effect its flavour. Some of these salts have become the flavour(s) of the month with each chef having his or her, own preferred brand.
Rock salt is mined from the earth. It comes in large crystals and is used as a garnish (e.g. oysters), in hand operated ice cream churns and in some pickling operations.