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 transaction 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 traveler pay for small purchase at the markets in dollars, for example, the vendor will usually slight you when the 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 give the money changer shops than banks. Money changer shops with post rate are the best place change dollars,Cambodian riel, Thai bath,Euro and another 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 Olympic Stadium.