Koshering salt, usually referred to as kosher salt in the US, is a variety of edible salt with a much larger grain size than some common table salt. Like common table salt, kosher salt consists of the chemical compound sodium chloride.
Unlike some common table salt, kosher salt typically contains no additives such as iodine, although some brands will include anticlumping agents in small amounts. Additive-free nonkosher salt is also readily available. The Salt Institute claims "Kosher salt contains no additives".
The term "kosher salt" comes from its use in making meats kosher by removing surface blood, not from its being made in accordance with the guidelines for kosher foods as written in the Torah, as nearly all salt is kosher, including ordinary table salt. One salt manufacturer considers the term ambiguous, and distinguishes between "kosher certified salt" and "koshering salt": "koshering salt" has the "small, flake-like form" useful in treating meat, whereas "kosher certified salt" is salt that has been certified as such by an appropriate religious body.
Manufacturing and use 
Rather than cubic crystals, kosher salt has a flat platelet shape. Salt crystals are forced into this shape under pressure, or grown flat in an evaporative process. Kosher salt is usually manufactured with a grain size larger than table salt grains.
The traditional use of kosher salt is for removing surface blood from meat by desiccation, as part of the koshering process for meat. The meat is covered with a thin layer of salt, then allowed to stand on a rack or board for an hour. The salt remains on the surface of the meat, for the most part undissolved, and absorbs fluids from the meat. The salt grains are then washed off and discarded, carrying away the fluids absorbed. (Compare this process to full desiccation of meat, which yields Salt-cured meat).
Use as a seasoning 
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.