East Japan (Tokyo, Yokohama) has a electrical current of 100v, 50Hz AC and the west (Osaka, Nagoya and Kyoto) uses 100v, 60Hz AC. We use plugs with two thin pins. Most devices such as phones and laptops will work fine, but appliances such as hair dryers and shavers can work slowly or may even get damaged without an adapter. Visitors from the UK and Europe will probably need to get an adapter, but those from North America may sometimes be OK as the shapes of the pins are identical.
Don’t forget to pick up a cheap adapter at a duty-free shop in an airport.