Most table salt sold for consumption contain additives which address a variety of health concerns, especially in the developing world. The identities and amounts of additives vary widely from country to country.
Iodine and iodide. Iodine is important to prevent the insufficient production of thyroid hormones (hypothyroidism), which can cause goiter, cretinism in children, and myxedema in adults. Iodine-containing compounds are added to table salt. Iodized salt is thus table salt mixed with a minute amount of potassium iodide, sodium iodide, or sodium iodate.
See full article at http://en.wikipedia.org/wiki/Salt#Additives_in_table_salt