Kosher salt, also sold as rock salt, is a type of coarse salt that is usually made without additives. The salt itself is not necessarily kosher, but takes its name from the curing process used on kosher meats. It is ideal for certain cooking projects and is preferred to table salt by many professional chefs because it has a more mild flavor, and the flaky crystalline structure of the salt helps it adhere to a variety of surfaces, from fish to margarita glasses.
Like all salts, this variety is a form of sodium chloride. It can be extracted from seawater by a direct evaporation process, or it can be mined from salt deposits under the Earth's crust. Table salt is heavily refined so that it has a precise square shape, and iodine is usually added during the refining process. Kosher salt is allowed to remain a more coarse-grained salt, meaning that the structure under a microscope looks like a series of cubes stacked on top of each other, rather than a single grain.