I heart soy sauce, I really do. A few times I have been tempted to drink it as it is but I think that would be the start of a slippery slope into condiment dependency. Before I really got into cooking I used any old soy sauce, the cheaper the better - oh how wrong I was...
If you really feel the need to read about the process of making and the history of soya sauce you can here
There are many types of soy sauce some vary from country to country such as Thai style, Chinese, Japanese, Hawaiian and Indonesian but even within Japanese cuisine there are many types of soy sauce. For those new to Japanese cuisine it can be a daunting business choosing the right one.
Probably the first division is between regular and reduced salt soy sauce. Which do you go for? well whichever you want baby, if you are worried about your salt intake go for the reduced one if not then regular is fine - there really isn't that much difference between them.
Japanese soy sauce is known as "shoyu" and is different to the regular Chinese varieties in that they taste slightly sweeter and some have a tang of "sherryness" about them, they can be divided into five categories:
Koikuchi originates in the Kanto region but now accounts for the majority of soya sauce found in Japan. These make good sauces for general cooking and seasoning of food a good exmple of this type of soy sauce is Kikkoman soy sauce
Usukuchi This is a lighter sauce popular in the Kansai region. The flavor is sweeter and not as rice as the dark soy sauces. In the UK they are commonly used for dishes which require a lighter more delicate flavour such as fish or dipping sauces.
Tamari This is a soya sauce made without the addition of wheat. Its commonly used by those following a wheat free diet. This is thought to be the one closest related to the original soy sauce introduced by China. Many people prefer this sauce for its rich dark taste and it makes an ideal partner for sushi.
Shiro also known as white soy sauce. This contains very little soybean and has a sweet taste. It is commonly used with foods such as Sashimi to highlight their flavours.
Saishikomi This is "twice brewed" soya sauce. It has a dark appearence and strong flavour, it is also known as "sweet shoyu"
For general cooking and serving with sushi you can't go wrong with the Kikkoman brand I mentioned before, its a good product at a great price. As for the other varieties you may want to experiment with them as you become more familiar with cooking.