Light soy sauce is the most used and is saltier and lighter in colour than its darker counterparts. It is normally used for cooking with fish or chicken or in a dipping sauce.
Dark soy sauce has been aged for longer and has a much more intense flavour. It is good for use in marinades and for cooking with dark meat.
Tamari is a special kind of Japanese soy sauce which is made without the addition of wheat and therefore is good for use in gluten-free cooking.
Shiro, in contrast, is made with more wheat than soya beans and is less widely available (look in Japanese supermarkets or online). Described as ‘white’ soy sauce, it is much more delicate and most commonly used as a dipping sauce for sushi.