Table salt is often "fortified" with additional ingredients to address specific public health concerns as well as ingredients added to preserve the appearance and stability of granulated salt.
Health related additives include iodine (as potassium iodide, sodium iodide or sodium iodate), iron (as ferrous fumarate), and fluorine (more common in Europe than US). In addition iodized salts may contain traces of dextrose used to stabilize the iodine salts.
Most finely granulated salts will contain some sort of anti-caking chemical. These include tricalcium phosphate, calcium or magnesium carbonates, fatty acid salts (acid salts), magnesium oxide, silicon dioxide, calcium silicate, sodium aluminosilicate, or calcium aluminosilicate.
Additionally, there may be trace minerals, commonly small amounts of magnesium and calcium halides and sulphates.