Is there a difference between Kosher Salt and Sea Salt? Can they be used interchangeably in a recipe?
Depending on the sea salt you have, it's probably less refined than the kosher salt. Celtic sea salt (with a grey-ish tinge) has a higher mineral content than your average highly refined salts and is especially rich in iodine. Not all sea salt is like that, though; some is just as refined as kosher salt.
Another factor is the size of the salt crystals. Kosher salt tends to have larger grains than your average sea salt, which tend to be small or very fine grains.
I would say in most recipes, they can be used interchangeably. If the recipe is using it to shake over the tops of rolls or pretzels, you probably want the larger size grains of salt, though fine grains will work (just with a different affect).
Sea salt has some impurities (which adds to flavor), and kosher should be straight NaCl. You can use them interchangeably based on weight. The flake size affects the weight by volume (for instance 1/2 cup of Diamond kosher salt weighs the same as 1/4 cup of table salt.) You'd have to figure out a conversion based on the products you have, or possibly you could find a conversion online.
Among salts, Kosher is the "saltiest" and sea salt is the least salty, depending on what variety it is.
Sea salt is harvested from nature and brings with it different flavors and mineral properties depending on where it is from. I have used Nepalese sea salt that you need 5 times more of to taste the salt, and Puerto Rican sea salt that is so flavorful that it almost tastes like broth.
Kosher salt is produced in a factory and tastes the same everywhere. The advantage is that it is cheaper than sea salt. It is better for recipes where you need a massive amount of salt, like a fish baked in a salt crust or something like that.
If you are going to use Kosher instead of sea salt, decrease the amount.
There is always a huge debate about salt, but in the end, for the most part, salt is salt. The main and most important difference in kinds of salt are the size and shape of the crystals. Some people maintain that some salt is "saltier" than others, when in fact what they are detecting is that one generally needs to use more salt composed of small crystals to achieve the same level of seasoning as with salt composed of larger crystals. Also, the shape of the crystals will affect how, and how quickly, it dissolves on the tongue, thus changing the perception of how salty it is.
While it is true that there are some specialty salts that treated with flavors (I have some great lime salt for margaritas) and some that are naturally high in specific minerals like iron that change the flavor slightly, in the end almost all salt is about 98% sodium chloride. What matters is how and what quantity you use it.
Agree with the above responses, but be careful when interchanging salts when baking cakes, cookies, etc. Sea salt usually piles up onto a measuring spoon quite differently then table salt or kosher salt does. I skip using sea salt for baking unless it's very fine or I'm using it purely for sprinkling on top.
Most recipes assume you're using table salt in their baked-good; but I find that kosher salt works fine if you allow it to gently round over the top of the measuring spoon, instead of level it off flat as you normally do when baking. Because of the bigger crystals there's supposed to be some calculation to find out how much more of it you'd need, but who has time for that. Haven't had a problem yet with my little technique.
Just to add a bit to the answers above, kosher salt dissolves really well, so it can be handy when you're adding towards the end of cooking, where as a large grain sea salt, which I use for most cooking because it's easy to handle, might take a bit longer.
Sea salt is also usually significantly more moist than Kosher or table salt, especially the very grainy, large flaked varieties. Finer sea salt is probably more compareable to table salt in moisture content. This would be a potential issue in baking applications, since additional moisture could skew your results. Because of the cost of sea salt, especially the large grained types, it's usually reserved for finishing or as an accent. Kosher is probably the main choice for "routine" seasoning, but for baking, table salt (non-iodized) would be prefered.