The official currency in Cambodia is the riel (CR), but the US dollar is widely accepted. Roughly equivalent to 4,000 CR, one dollar is a very common price for almost anything in Cambodia. In Phnom Penh, it would buy a sandwich or a dumpling soup. In Siem Reap, it could pay for one’s fare to the other end of the street or three wooden bracelets sold by English-speaking schoolchildren.
The Vietnam dong (VND) or the Thailand bath (THB) are accepted currencies as well, though only in places close to their respective borders.
Although there are establishments which accept credit or debit cards and there are already some ATMs in major cities, it is always best to carry cash in small denominations in Cambodia, preferably in US dollars.