Soy sauce has been an essential flavor enhancing ingredient for over 3,000 years. It has two major types - light and dark. Brewed directly from fermented soy beans, wheat flour, water, and salt, light soy sauce is lighter in color, saltier in taste but more savory or umami in flavor (xian in Chinese). It is the main soy sauce used for seasoning. Blended from light soy sauce with caramel and aged for a longer period of time, dark soy sauce is thicker in texture, darker in color, but a little sweet, less salty and less savory or umami in flavor. It is mainly used for coloring and stewing dishes. Some ingredients may be blended into the dark soy sauce to incease the savory flavor, such as mushroom soy sauce, shrimp soy sauce, etc.
Confusion regularly comes among Chinese soy sauces and Japanese soy sauces. Roughly, there are three types of Japanese soy sauces:
Light soy sauce does not mean less sodium soy sauce at all. If a recipe calls for light soy sauce, try to determine if it's a Japanese or Chinese recipe. If the recipe is Chinese, light soy sauce means Chinese light soy sauce or regular soy sauce. If it is Japanese, it may mean usukuchi soy sauce.
Less sodium soy sauce is usually more expensive. It is because the process to produce low sodium soy sauce requires the same initial amount of salt and the same procedures as for regular soy sauce. The difference is an additional step at the production end to remove certain amount of sodium (40% usually)