At the most basic level, all salt is simply sodium chloride. The difference is in texture and the addition or lack of minerals.
Table salt is made with very fine grains. Minerals are removed, but it usually includes an anti-caking agent, such as calcium silicate. If it’s iodized, iodine has been added to prevent thyroid diseases in regions where people’s diets are low in iodine.
Kosher salt is made with larger grains. It’s not necessarily a kosher product, but it’s used in the process called koshering — removing blood to purify meat — because the larger grains dissolve more slowly and are better at removing moisture from meat.
Cooks like kosher salt because the larger grains make it easy to grab a pinch. It also doesn’t have anti-caking agents, which could make brines cloudy.
You can swap the two in cooking, but because table salt is finer, more grains will fit in a measuring spoon. So a recipe formulated to use kosher salt might be saltier if you make it with table salt.
— Kathleen Purvis