There are differences in salt.
Kosher Salt generally is only sodium chloride (NaCl), without iodine or other trace minerals or additives, and is coarse grain. It is called kosher salt due to being large grained and suitable for the kosher blood draining process (ensure that kosher salt is certified kosher if needed for that purpose).
Table salt is generally mined and ground into a small grain. Some (not all) table salt is fortified with iodine, an essential mineral.
Iodine is an essential trace element, that is important for thyroid hormones, which are important for various regulatory processes in the human body. It may also have other physiological functions, such as immune response.
Iodine usually occurs as a salt and therefore as iodide or in the case of being an additive to table salt, as iodized salt.
Iodine is present in many foods, and in higher concentrations in:
- yogurt, milk, and other dairy
- fish and shellfish
Dried nori seaweed has the highest concentration of iodine of any naturally occuring food.
The WHO recommends the use of potassium iodate, though the US FDA has approved potassium iodide and cuprous iodide. Most countries have adequate iodine in their population, with some exceptions. For example, Vietnam has inadequate consumption of iodine though the rest of ASEAN has adequate consumption.
That said, a diet with healthy doses of the food listed above should not need iodine supplementation.
Sea salt is evaporated salt water, and as such contains trace minerals including magnesium, calcium, potassium, etc. However, it does not contain iodine, as table salt does. It is generally coarse grained like kosher salt.
Sodium Chloride by Weight and Volume
Because of the grain size, sea salt and kosher salt have less sodium chloride per volume (e.g., teaspoon). This means that using coarse grain salt (sea or kosher) instead of fine grain salt (table) in cooking should take the grain size into account.
Some chefs recommend adding 1/4 teaspoon of coarse salt to every 1 teaspoon in a recipe that is calling for or assuming table salt. However, depending on the coarseness, the difference in weight may be as much as twice as much.
All salt types contain approximately the same amount of sodium by weight, around 40%, and there is no real health benefit between the different types of salt based on sodium content.