There is a wide variety of soy sauces. In grocery stores where I live there are generally only two "types" printed on the labels though: "Japanese" and "Chinese", but that doesn't explain much.
In cooking shows from the UK, I've seen mention of "light" and "dark" soy sauce, which apparently matches the varieties in their grocery stores, but we don't have those here.
The last one I got labelled "Chinese" doesn't taste well IMHO and I can't pair it with much food.
The salt content can't be good. But there's Japanese "Tamari" which supposedly has less salt.
I believe the sauce we get in small cups at sushi places sometimes is sweetened with sugar.