Only citizens from Malaysia, Singapore, Laos and The Philippines can visit Cambodia without an entry visa (valid for visit within one month). All other citizens are required to get an entry visa before departure or a pre-approved entry visa issued and complete before arrival at a Cambodian Embassy or consulate all over the world, or through Internet : http://evisa.mfaic.gov.kh/
A Visa on Arrival is an easy way to obtain your visa without having to chase down embassies and consulates. The entry visa will be stamped on your passport as you pass through the immigration checkpoint (bring a pass sized picture). This visa is valid for one month and it can be extended once you enter Cambodia.