The Japanese currency is called "yen" (en, 円). It is easiest to think of 100 yen as the equivalent of one US dollar, adjusted with the exchange rate, of course.
In Japan, cash is the most widely accepted form of payment, which may catch you off guard if you normally use debit or credit cards for common transactions in your home country. Please be aware that many services, including shops, train stations, and taxis, may not be able to accept your card, so be sure to acquire as much cash as you need at the beginning of your trip. It pays to be prepared!
Foreign currency exchange services can be found at airports, large banks, and post offices (Japan Post Bank). Note that most services are only open during regular business hours and may be closed on weekends.
Not all Japanese ATMs provide service for international cards. See our Using ATMs and Credit Cards page for more information.