Kosher salt (or more correctly, koshering salt), is a term that describes one of the most commonly used varieties of edible salt in commercial kitchens today. Kosher salt has a much larger grain size than regular table salt, and a more open granular structure. Like common table salt, kosher salt consists of the chemical compound sodium chloride. Unlike common table salt, Kosher salt typically contains no additives (for example, iodine), although kosher salt produced by Morton contains sodium ferrocyanide as a free-flow agent. The term kosher salt is restricted to North America; in other parts of the world, it is called (coarse) cooking salt.
Kosher salt gets its name not because it follows the guidelines for kosher foods as written in the Torah (nearly all salt is kosher, including ordinary table salt), but rather because of its use in making meats kosher, by helping to extract the blood from the meat. Because kosher salt grains are larger than regular table salt grains, when meats are coated in kosher salt the salt does not dissolve readily; the salt remains on the surface of the meat longer, allowing fluids to leach out of the meat.
Kosher salt can be used in nearly all applications, but it is not generally recommended for baking with recipes that use small amounts of liquid (wet ingredients). If there is not enough liquid, the kosher salt will not dissolve sufficiently, and this can result in small bits of salt in the resulting product; in certain applications this is undesirable. In recipes where there is enough liquid to dissolve all the salt, table salt can be replaced by kosher salt, but the volume must be adjusted. Because kosher grains occupy more volume (for equal weight) the volume of kosher salt should be increased. Because kosher salt grains can vary in size considerably from one brand to another, it is recommended that one check the box for a conversion guideline, which is generally provided. If there is no guidance provided, twice as much kosher salt (by volume) to replace table salt serves as a rough estimate. Another reliable technique is to use an equal weight; a gram of kosher salt is equivalent to a gram of table salt.
Chefs often prefer kosher salt because its texture allows the chef to pinch a larger quantity of salt and evenly sprinkle the flakes on food. Also, because of the absence of iodine, kosher salt tends to make flavors cleaner and brighter than iodized salt, which has a slightly metallic flavor. The Food Network's Alton Brown and many others generally recommend the use of kosher salt for this reason.