Soy sauce is a basic ingredient in Japanese as well as many other Asian cuisines.
In Japan, there are basically four types of soy sauce: regular dark, light or usukuchi, reduced sodium or genen, and tamari, which are the rather syrupy dregs of soy sauce at the bottom of the barrel. The first two are the ones most commonly used for cooking. Reduced sodium is of course used by people with high blood pressure concerns. Tamari is never used for cooking - it's usually used as a dipping sauce, for sashimi and such.
Which soy sauce to use, dark or light, depends on what region of the country you're from. In Kyo-ryori, the distinctive type of cuisine that originated in the old imperial court of Kyoto, dark soy sauce is considered to adulterate the flavors and colors of the ingredients, so usukuchi or ligh soy sauce is used. Beware though - even though it's lighter in color, light soy sauce is not lower in sodium. If anything, it's usually higher in sodium.
There are many "designer" brands of soy sauce in Japan too - sort of the way there are several high-class varieties of balsamic vinegar in Italy. The usual price for a liter of soy sauce is maybe about US $4-5, but I have a bottle that cost 10 times that, of a special kind of soy sauce that is hand-made and aged or something. To be honest, I can't detect that much of a difference between that and cheaper good brands.
A good soy sauce should have plenty of flavor. The best way to see what you prefer is to just taste. I usually get the Yamasa brand, since it's widely available Kikkoman makes its soy sauce locally throughout the world (a bit of trivia: Kikkoman is one of the oldest, if not the oldest, food producing company in the world, having been founded in the 17th century). and is also decent in flavor.
You don't have to refrigerate soy sauce, but do screw the cap on tight and keep in a dark place. And always use sparingly.