The question over what kind of salt to use, and why there's such a difference in price between the various varieties, comes up often.
First Anne asked what the difference was between Morton's and sea salt. Pat responded that sea salt is expensive stuff, and suggested using kosher salt for finishing a dish instead.
The easiest way to think of salts is to break them down into three categories: table salt, regular sea salts and kosher salts, and finishing salts. I keep all three around: table salt for baking, kosher/sea salt for cooking, and finishing salt for serving. Table salt (the fine white stuff you're used to seeing in a round cardboard container) usually has additives like iodine (to prevent thyroid disease) and an anti-caking agent to prevent lumping in humidity. Table salt without iodine is available, especially in health food stores. We use Hain without iodine for baking and go through a 26ozer once a year at the most. Most brands costs under $1.
Sea salt is not necessarily expensive - the specialty varieties like Fleur de Sel are expensive, and we'll talk about that in a moment. For cooking, regular old sea salt is superior to table salt. A 26.5oz container of La Baleine Coarse Sea Salt will run about $3.00. Considering the amount of time it takes to blow through this quantity of salt, you really won't notice the difference unless you're pinching pennies big time.
Like sea salt, kosher salt is preferred by chefs for cooking because it has no additives and is coarsely grained and leaves more salty flavor-bursts in the food. The flakes of kosher salt are a bit lighter than sea salt, so kosher salt is often preferred for seasoning sautés, brining meats, and salting water. Experiment; choose your favorite. It usually comes in large boxes (48oz) and runs about $2.
Finishing salts are the upper echelon of sea salts and should be used only on finished food - not while cooking. I prefer Fleur de Sel, which is made from young crystals that bloom naturally on the surface of salt evaporation pond. The consistency is light and flaky - coming across a little nugget of Fleur de Sel in your food is a pleasure. It brings out the flavor of the food without dominating it with an overly salty flavor. It costs much more than table salt or kosher salt (about $10 for 8 ounces), but once you try it, you'll see why it's worth the extra expense.
Beyond the above mentioned salts, there are dozens more. But so much choice can simply confuse. The most you need is three varieties, and even that, I admit, is a bit excessive. If you want to read more about salt, here is an article I wrote - Salts of the Earth (Washington Times, October 5, 2005)
And if you really want to read more about salt, I highly recommend Mark Kurlansky's Salt: A World History.