Kosher salt may or may not be sea salt. Kosher salt is not blessed, nor does it have any particular religious significance. Most salts are kosher, but that doesn't mean they're kosher salt.
Kosher salt is salt which has been, crushed, milled, flaked, harvested, or otherwise made so that it (a) sticks to meat, and (b) does not dissolve easily. In point of fact, it's usually (Diamond and Morton) flaked. This is because using salt to dry the blood out of freshly slaughtered meat is part of the process in making kosher meat. Because it doesn't dissolve as easily it can be washed off when the koshering process is over -- and not leave too much salt in the meat.
Kosher salt may contain anti-caking additives such as sodium prussiate, but is not iodized. This is because kosher salt is frequently used for long contact, and iodine tends to turn food over time.
Unlike other caking agents, sodum prussiate will not cloud liquids -- so, if you like you can use Morton for pickling without clouding the jar. Compared to Morton, Diamond seems to taste a little clearer. Compared to Morton, Diamond provides almost exactly the same amount of sodium, weight for weight. The difference in sodium is volume per volume -- obviously Morton is denser. I taste tested both of them a couple of months ago based on something Shel wrote, and IMO Diamond, at the same seasnoning level, had a clearer, "cleaner" taste than Morton.
Other brands with some distribution are Regal, Redmond, David's and Sonoma. Regal is very coarse and well priced. Redmond is nothing special and incredibly over priced. David's is nothing special. Sonoma is a designed crystal, and supposedly the ultimate salt for rubs. Given Shel's degree of maven, it's probably worth it for him to look it up. I haven't got around to it.
The standard conversion for sodium is:
1 volume of table salt = 1-1/2 volumes Morton kosher = 2 volumes Diamond kosher. They are equally salty weight for weight.
Because it stays on the surface and is so resistant to dissolution, kosher salt is ideal for rubs -- and is also good for sprinkling by hand at the table if, for some reason, it's desirable for the salt to stay visible. Other than that, it's just salt.
There are subtle differences between various salts which become apparent during certain uses -- almost all of them dry. These differences don't depend on where or how the salt was sourced, but rather on impurities in the "gourmet salts." In the case of color salts like "pink," or "gray" the difference is soil.
Salt is known as sodium chloride, NaCl, and it's just that -- sodium chloride. So, when it gets down to it, most salts are just salt. If you use anything other than ordinary table salt in any dish with strong flavors where the salt's texture or the subtle flavors imparted by the impurities are overwhelmed the differences are lost.
An example where a good salt will stand out is as a finishing seasoning on a relatively simple food -- a baked potato with butter and pepper, for example. My particular favorite finishing salts are fleur de sel, and Hawaiian pink and black.
I used to write about kosher salt four or five times a year when I was active in online barbecue forums where the subject comes up frequently. All of this stuff is off the top of the my head, but the information is widely available from a number of sources. IIRC the online Cook's Thesaurus has a very good presentation.
Watch out for recipes from sources like Michael Chiarello with a financial stake in a particular salt. It tastes a lot better, makes a bigger difference, and is more often "worth it," when you own the company. That said, if you think $8 pound salt makes a difference in your habanero soup, enjoy yourself. Because I can't taste it, doesn't mean it isn't there.
Hope this helps,