This is the official currency of Cambodia but it's less used than the dollars in the hotel and for larger transactions, more use than the provinces, and for smaller transactions everywhere.The riel comes in denomination of 50,100,200,500,1000,10000...ect. It's always a good idea to have some riel on hand. when travelers pay for small purchases at the markets in dollars, for example, the vendor will usually slight you when they give you change in riel. It's good to have 500 and 1000 riel notes to pay the moto or taxi drive and to leave for a tip.
The exchange rate fluctuates but has lately been around 4080-4200 riel to one U.S dollar. For currency exchange, betters rates are given the money changer shops than banks. Money changer shops with post rates are the best place to change dollars, Cambodian riel, Thai bath, Euro, and other currencies. Always count the money they give you and check it on their calculator. One no-nonsense shop in Phnom Penh is Lyhour Group.
Lyhour shop exchange rate in Phnom Penh: The located on the north of the Olympic Stadium.