Soy sauce is made from fermented soybeans mixed with a roasted grain- usually wheat, but sometimes barley or rice. Its use was documented in China more than 3000 years ago. The Japanese adopted the idea only about 400 years after it was first used in China. They experimented with their own techniques, and arrived at a product that was slightly sweeter and more refined than the Chinese types.
It imparts a slight saltiness and adds depth to the flavor of a dish without masking the essential flavors of the main ingredients.
Dark soy sauce is dark and slightly thick with a full-bodied flavor. It is used when a deep color is needed. Generally dark soy has less salt and may be slightly sweeter than the lighter types.
Light or "White" soy sauce is thinner, saltier and lighter in flavor. It is used as a condiment and in cooking where its light color will not spoil the colors of the ingredients, particularly seafoods. Like dark soy, it can be kept for many months without refrigeration.