There are two types of salt: salt mined underground from deposits, and salt collected from evaporated seawater. In both cases the salt is primarily sodium chloride, but flavor differs depending on the minerals present in the earth or water from which the salt was taken. Individual salts also vary due to differences in geography and bodies of water.
Kosher Salt has coarse, irregular crystals and is typically additive free. While traditionally used in the preparation of kosher meats becasue of its ability to draw out a miximum of blood, it is used by chefs daily. Its large crystals make it easy to grab a pinch and it has a fresh, clean taste without any discernable mineral notes.
Sea Salt is harvested everywhere in the world and can come in many colors, from bright white or gray (from the bottom of salt ponds), to red and black (Hawaiian). Some sea salts are cut into light flakes, some are fine, some are coarse, and some are shaped into rounds. The bright briny flavor of these salts adds zest to all your salads. Processing sea salt is more labor intensive than mining salt, and therefore the cost is much higher.
Table Salt which is mined from the earth and is generally formed into very fine, granular, pure white crystals. It is often fortified with iodine for nutritional reasons and usually contains an additive to keep it free flowing in damp weather. When a recipe calls for salt, it usually means table salt, unless otherwise noted. Because table salt is fine grained more crystals will fit when measured so that 1 teaspoon of table salt will weigh more than 1 teaspoon of coarse salt. If you are substituting one for the other remember that a teaspoon of table salt will add more salt flavor than a teaspoon of coarse salt. Table salt also has a slighly saltier edge than kosher or sea salt.
There are 5,370,000 crystals in a pound of table salt and 1,370,000 crystals in a pound of kosher salt, give or take a crystal or two. Source, Good Eats by Alton Brown