Japanese electrical outlets are 100 volts. Electrical current varies depending on where you are in Japan. Eastern Japan is typically 50 Herz, while Western Japan is 60 Herz. The outlets are designed for two prongs. Generally, American-made electronics (110/60) will have no problems with the Japanese electrical systems, though you might want to consider bringing a converter if you have sensitive or expensive equipment. European items will generally require conversion (although some hotels can accommodate European electronic items).
If you buy electronics in Japan, verify that the items are made for use with other electrical systems.