The currency of Cambodia is the Riel, although US Dollars is used at most places frequented by tourists and can be used interchangeably for Riel just about everywhere else. While the Riel is theoretically convertible, it isn't commonly traded. Prices at hotels, restaurants and tourist sights are usually in US Dollars. However, businesses in Cambodia don't keep US coins on hand, so small change will usually be given in Riel, at a common exchange rate of 4,000 Riel to one US Dollar.
There are currency exchanges throughout the major tourist destinations, but changing money is more or less unnecessary if you're carrying US notes in small denominations. Automated Teller Machines (ATM) are now quite common. You'll find them in most tourist areas as well as airports. With a very few exceptions, they dispense US Dollars, not Riel. Those that do dispense both will give you the option of what you want. Note that as of mid-2010, all Cambodian ATMs charge an additional US$ 4 for foreign ATM card use.