The primary difference between soy sauce and tamari sauce is that traditional soy sauce contains wheat, whereas tamari sauce often doesn't. Consequently, tamari sauce is usually the safer option for those following a gluten-free diet, as is commonly the case with sufferers of celiac disease.
Both soy sauce and tamari sauce are byproducts of fermented soy beans, a shared origin that makes them generally similar in both color and taste. However, while traditional soy sauce is ubiquitous throughout Asia, tamari sauce is more specific to Japanese cuisine, being a byproduct of miso paste. While soy sauce is brewed, tamari is collected from the liquid forming atop the miso as it thickens. This is something like liquid whey separating from curd during cheese-making.
Despite broad similarities in taste and appearance, there are recognizable differences between soy and tamari. Tamari tends to be richer and darker than traditional soy, and it typically has a lower overall salt content. These features make tamari more ideal for dipping than soy, which is frequently thinner and more biting. Tamari is also the more classic accompaniment to sushi, and most restaurants have it available for patrons who ask. Although most tamari varieties are either gluten-free or low in wheat, the consumer should still check the label to make sure.