Kosher salt is made from evaporated brine. It has no additives or iodine. It is popular with cooks because it's course texture makes it easy to pinch and sprinkle and is less "salty" because of it's size, so that it's hard to over-salt foods, and the coarse texture makes it adhere to meats before cooking. To use kosher salt in any recipie that is configured for table salt, you double the amount. It's also known for it's fast dissolving in liquid, so the effect on taste can be immediately checked.

Table salt, usually iodized, is great for baking where the fine grained crystals are easily measured.