Soy sauce is made from soy beans, wheat and salt, and fermented for several months.
Koikuchi shoyu (dark colored soy sauce), below, was developed in the east of Japan (Tokyo area) and it is dark colored with a slightly fruity flavour that reduces fishy and meaty smells in cooking.
Usukuchi shoyu (light colored soy sauce), below, originally favored in the west of Japan (Osaka area), has a lighter color and saltier taste than koikuchi.
Substitution: There is no substitution for soy sauce. For Japanese cooking I would recommend you to use Japanese soy sauce because I can differentiate Japanese and other kinds of soy sauce. However, if you cannot tell the difference, you may not need to buy a Japanese soy sauce especially if you don’t plan to cook Japanese food often.