Chinese soy sauce is made only of soybeans.  It’s what we in the U.S. are most familiar with.  Japanese soy sauce is called Shoyu.  Shoyu is made from soy beans, roasted wheat, water and salt.  It is a little sweeter and less salty than Chinese soy sauce.  Shoyu is used as an all-purpose cooking soy sauce.  Tamari is a non-wheat product made by drawing off the liquid content of soybean miso.  Since genuine Tamari is a non-wheat product, it has a distinctive aroma as well as thicker texture, deeper color, and stronger taste. It is often used for dipping raw fish (sashimi), sauteing teriyaki, and other food processing. 

The Chinese, particularly in the south, use both light and dark soy sauce. The dark is aged longer, and toward the end of the processing is mixed with bead molasses, which gives it a darker, caramel-like hue. Similar to red and white wine, since as a rule, dark soy flavors and colors heartier dishes, particularly those with red meat.  Light soy sauce is more appropriate with seafood, vegetables, soups and in dipping sauces.