As others have pointed out, kosher salt is just salt without additives. It's fairly coarse, which makes it ideal for picking it up with your fingers and dropping it into your pot without losing any of it. You quickly develop a feel for how much salt you are adding, and you can easily work out how many large pinches make up 5 g, for example. Other than that, for cooking, any salt will do as far as taste is concerned, kosher or not.
There are a few occasions where you want to use additive-free salt though (kosher or any other pure salt). Pickling is one of them. Iodised salt tends to discolour things. In particular, pickling garlic with iodised salt causes the garlic to turn dark green/blue. This doesn't affect the taste or texture, but it doesn't look very appetising. Sometimes, even the dissolved minerals in tap water are enough to discolour garlic. (Demineralised water plus kosher salt is the way to go in that case.) Cheesemaking is another thing where you definitely need pure salt, without iodine or anti-caking agents.
If you find it difficult to get kosher salt, look for pickling salt or salt for cheesemaking. Both are NaCl without anything else, as pure as you are going to get (short of laboratory-grade NaCl).
Don't cook with fine table salt even though it tastes the same—it's a pain to dose correctly. Coarse salt is the way to go.
