Electricity in Tokyo runs on 50 cycles/100 volts (60 cycles/100 volts in Osaka). Small U.S. appliances will work in Japan but will be slightly less efficient. For items such as small kitchen appliances, lamps, vacuum cleaners, etc., this difference in efficiency will not be problematic. For larger appliances where 100% efficiency is required, transformers can be purchased for 8,000 to 10,000 yen.
Appliances from Europe will not work in Japan, so you would require a transformer or they should be purchased or leased upon arrival. There are 2-pin plugs used in Japan but not 3-pin plugs in Japan, and, similar to those used in the United States, are used instead.