Japan's mains electricity supply is 100V, at either 50 or 60Hz depending on where you are in the country. The plugs and sockets are similar to US ones, but slightly different.
Using your devices in Japan
Many modern devices will accept universal 100V, but check first before plugging in. Most will state what voltages are acceptable on them somewhere; if not, don't risk it. Things like laptops and phone chargers that are designed for travel are usually universal voltage. Things like cooking equipment (including kettles) and heaters are not.
Some hotels can loan you a charger, or you can use your laptop's USB ports, or just buy a Japanese one at an electronics store for a few hundred yen.
Using Japanese devices in your home country
The same goes for Japanese 100V devices you might want to take home. Many will work on 120V or 230V, but not all. You can buy voltage converters, but finding one that produces 100V can be tricky as most are for 120V/230V.
A note about voltage converters. Electronics are usually designed to accept ±10% of their rated voltage. For example, a 100V appliance will accept 90V to 110V (-10% to +10%). Beware that many of the converter on eBay and Amazon say "100/110/120V" or something like that. Sometimes they put out 110V, sometimes 120V. Neither is any good for stuff that needs 100V, because even though 110V is within the +10% margin, if your mains supply is +5% as well then the total will be +15%.
Also look out for very small, high power (1000W+) converters. If you check the fine print they are only for things like electric cooking hobs. They will blow up pretty much anything else. A proper 1000W 230V step-down converter will weigh several kilos.