One of the primary differences between kosher salt and table salt is that table salt contains an anti-caking agent, such as iodine or calcium silicate, while kosher salt does not. Kosher salt and table salt are also different in grain size.
Sodium ferrocyanide is another anti-clumping agent that is added to table salt. The grains of table salt are small because the salt has been milled. Kosher salt grains are large and crystalline in structure, as they have been kept in their original state.
Kosher salt did not get its name from the fact that the salt itself conforms to Jewish dietary rules. The salt is actually used to make meat kosher by drawing blood from the surface of the flesh. Meat absorbs salts that have smaller grains, which would not make for effective koshering. Kosher salt absorbs surface blood and can be washed off easily without giving the meat an extremely salty taste.