The unit of currency is the North Korean won (KPW), though most travellers will never even see them. Banknotes come in denominations of five, 10, 50, 100, 200, 500, 1000, 2000 and 5000KPW and coins come in denominations of one, five, 10 and 50 chon, as well as one won. Visitors can pay for everything with euros or Chinese yuan (but bring small change of both; big notes can be impossible to change). US dollars and Japanese yen can also be exchanged, but generally at poor rates. While you're unlikely to use the won, it may be possible to get some from your guides as a souvenir (it’s officially illegal to take it out of the country, so hide it deep in your luggage).
Credit cards are completely useless everywhere in the country, so bring as much cash as you’ll need with some leeway for any unexpected expenses. Bring your cash in euros or yuan. Travellers cheques are not usable in North Korea and there are no ATMs anywhere in the country.