The Cambodian riel is the official currency, but US dollars are universally accepted in Cambodia. Most ATMs dispense US dollars instead of riels. They charge for a withdraw a fee of 3-5$. Canadia bank doesn't charge a fee. While there are sufficient ATMs in the major tourist areas of Sihanoukville, Siem Reap and Phnom Penh which dispense US$ it may be wise to bring your own supply of US$1, $5, $10 and $20 bills to avoid problems changing larger denominations of $50 or $100 notes. US dollar coins buy nothing but confused looks.
Cambodian riels are used instead of US coins, at a fixed rate of 4000 riel/dollar for calculation convenience. So USD 1,50 is one dollar plus 2000 riel. There are also 20000 riel bills (5 USD), but higher amounts are usually paid in dollars. Because the actual exchange rate is higher (about 4200 riel in January 2010), try to get rid of as many riel as possible towards the end of your trip (also because the riel may be hard to change abroad). Near the Thai border (especially Battambang, Koh Kong, and Poipet) Thai baht is also accepted; further east (including Siem Reap) baht can easily be exchanged, but cannot be spent - except at uncompetitive rates. Likewise Euro can easily be exchanged, but cannot be spent - except at uncompetitive rates. Banks give the best rates, avoid money changers at markets or on the street. Torn foreign currency notes can be difficult to exchange. It's acceptable to check each note and ask to have them changed if you aren't happy with the quality, even in banks.
If you're planning on heading out off the beaten track, you need to take enough US dollars to get you back to a point where you can get more.
In many of the larger towns one or more of the local banks operate as Western Union Money Transfer agents.