Soy Sauce - Common Japanese Ingredient

Soy sauce (shoyu) is a Japanese cooking ingredient made from fermented soybeans. Although there are many types of soy sauce, all are salty and earthy tasting brownish liquids used to season food while cooking or at the table. Soy sauce forms an important part of many Asian cuisines. Different countries make very different soy sauces, and it is rarely appropriate to substitute one for another (e.g., Chinese for Japanese). Japanese soy sauce is called shoyu. The English name soy came from the word "Soi" (そい) of Satsuma dialect Japanese.
Soy sauce comes in two varieties - 'light soy sauce' (生抽), which is a thin light brown liquid, and 'dark soy sauce' (老抽), which is essentially the same, except with caramel added for coloring and thickening. Dark soy sauce is used when it is desired that the dish be colored, or when used for a dipping sauce.
Soy sauce contains a small amount of naturally occurring MSG. It is also extremely salty, so it is not a suitable condiment for some people. Low-salt soy sauces are produced, but it is impossible to make soy sauce without using some quantity of salt.

credit : http://www.japan-101.com/food/dining_soy_sauce.htm