Japan uses 100 volt 50mhz/60mhz (west/east, respectively) for electricity. They do not use 3 prong plugs at all. Contrary to what I heard before departure, the plugs are grounded/polarized, so American electronics will fit (at least here anyway, and in Fukuoka). Most modern electronics can handle a range of voltages and frequencies (usually 100-240 volts and 50/60mhz). All of my electronics (laptop, external hard drive, electric razor, etc.) are compatible with Japanese electricity except my battery charger. Electronics that get hot easily already will get even hotter on Japanese electricity, such as my external hard drive, which was basically a space heater to begin with.
Also, I forgot to mention in my previous post about units that Japan uses the 24 hour clock, military time, whateveryouwanttocallit. So, basically, 1pm is 13:00, 8pm is 20:00, etc.