A lot of the recipes I post here, mostly the Chinese or Thai recipes, will state: Light Soy Sauce, Dark Soy Sauce, or just Soy Sauce. Here is a breakdown and this is not in any way, a official description or use.
Light Soy Sauce – Normally more salty, thinner, but a lighter color so the food being cooked is not turned as brown from the sauce. This can be used in place of Dark Soy Sauce, but use less due the more profound saltiness.
Dark Soy Sauce – Aged longer, can contain molasses or caramel, thicker generally but less salty than Light Soy Sauce, cooking methods bring out the flavor of the sauce. A darker color in foods is the result with using this soy sauce.
Soy Sauce – General purpose, often used as a dipping sauce, marinades, and dressings.