Many people have only heard of the term "salt" as it pertains only to our food. There are a variety of different edible salts, of course, but did you ever wonder what the difference is? Aside from that, the term "salt" has a well-defined chemical definition, which includes a variety of compounds, other than the well-known NaCl.
A salt (in chemistry) is made up of some negatively charged ions and some positively charged ions, but the overall charge is neutral. There are a few different varieties of salts, such as basic salts, acid salts and neutral salts, depending on what type of ions they release when dissolved in water.
When salts are dissolved in water, they are called electrolytes. You have probablly heard this term used in Gatorade commercials. Salts are important to the body for many reasons, so replenishing them, along with water, is important when excessive perspiration makes the body lose alot of water and salt.
Table salt is also called edible salt, halite, or NaCl. It is made of Sodium (Na+) and Chloride (Cl-) ions bound together. Table salt is a relatively pure substance, made of a majority NaCl molecules; it often has iodine added though. It is obtained from evaporation of seawater, or by mining rock salt (the mineral form of NaCl). the Rock salt was formed by evaporation of enclosed, ancient lakes.
Sea salt is also mostly NaCl, but it does have a mineral content, resulting in a different flavor than table salt. The mineral content (and flavor) can vary based on the location it was harvested from. It tends to have a much coarser texture as well.
Kosher salt is also pure NaCl, but usually does not have any iodine added, making it the most pure salt you can purchase at the grocery store. Kosher salt also has a large grain size, but the structure is a bit different from other NaCl salts. It gets its name due to its use in making meats kosher, by drawing the blood out during processing.
Leave it to Morton's to make up a bunch of other kinds of salt I have never heard of! Check out their website for more salt types; they are basically variations of typical NaCl, I believe! They have a nice section on where salt comes from, too.