Table salt is a form of salt that is designed to be used in cooking and at the table. This form of salt is refined to remove impurities and it may include some additives, depending on where it is processed. It is distinct from unrefined salt, which is allowed to retain its impurities. Unrefined salt can also be used for seasoning foods, and some are actually highly prized precisely because of the impurities they contain.
One source of sea salt is brine, with the salt being produced through evaporation. Table salt can also be produced from rock salt, in which case evaporation is not necessary for processing. The salt crystals are processed to remove any impurities that may be present and allowed to recrystallize. This process gives the salt a white to clear color, and allows the crystals to reform in very regular shapes. During processing, the salt may be iodized, meaning that iodine is added, and it can also be treated with anticaking agents to prevent it from clumping.
Iodized salt is sold in many regions to ensure that people get an adequate dietary supply of iodine, a necessary nutrient. Iodine deficiencies can lead to goiters and other health problems, and providing iodine in salt is an easy way to ensure that everyone in a population gets enough. Salt that has not been iodized is also available for sale, and it is usually required by law to be clearly labeled so that consumers are aware that the salt does not contain the nutrient.
Anticaking agents are not necessarily always added to table salt, but they are a common addition. Even with anticaking agents, salt can still tend to clump. Various tactics can be used to reduce clumping including mixing grains of rice in with the salt and using specialized containers that are designed to prevent moisture from getting to the salt.
Unrefined salt is sometimes sold as “sea salt” or “bay salt.” In fact, table salt can also be derived from the ocean, it's just been more heavily processed between the water and the table. Unrefined salts can be gray, white, pink, or clear, depending on the impurities they contain, and the crystals are often irregularly sized. Some people claim that unrefined salt has a distinctly different flavor, and food researchers have demonstrated that the irregular crystals cause it to perform very differently in food.