The difference between Kosher salt and regular table salt (other than the size of the grain) is the iodine content. Table salt is iodized as a dietary supplement. Kosher salt is not. If you know the characteristics of the tastes, iodized salt can have a bit of a metallic taste. Professional Chefs use Kosher salt because they want seasoning of pure salt without any additives. But most non-professional cooks probably won't taste a difference.
'Sea Salt' is made from evaporating sea water to leave just the salt. Most salt in America is what's called 'mined' salt - salt that comes from underground mines. Even though mined salt may have been deposited from ancient evaporated seas it's still considered 'mined' salt and not sea salt. Sea salt is more expensive to produce as its more labor intensive and most Chefs use it as a finishing salt and not as an ingredient salt. And sea salt can impart differing flavors (some very distinctive) based upon the sea from whence it came, and professional Chefs want just the salt.
Lastly, be careful when using sea salt as it is a bit saltier by volume that is regular Kosher salt or table salt. One teaspoon of table salt typically does not contain the same amount of NaCL (sodium chloride) by volume as does sea salt. You can't substitute 1:1 in a recipe as the sea salt is 'saltier'. Table & Kosher salt by volume is about 12% NaCL while sea salt can be as high as double that.
Lastly, salt has an expiration date due to it's anti-caking characteristics (and because the government mandates it). Anti-caking agents can break down over time and can lose it's anti-caking powers. Folks then try to use salt that is caked up really bad and thinks its gone bad. Salt never goes bad due to its lack of innate moisture, hence the anti-caking chems.
And never salt your steak too far in advance. Salt can break down cell walls and make a raw steak weep it's juices. You may end up with a tougher steak.
