Most visitors to Cambodia will need a visa. You can obtain one in advance from a Cambodian embassy or consulate, or you can apply for one on arrival in Siem Reap or Phnom Penh. Most people opt for the visa on arrival, so the queues to get the visa can be quite long.
To obtain a visa on arrival, you'll need to complete an application form and attach a passport photo of yourself to the application. Your passport will need at least one full blank page to paste the visa into, and your passport will usually need to be valid for at least six more months from your date of arrival. The fee for most visa types is US$ 20.
You can also apply for a Cambodian visa online. It takes three days to process the e-visa and you will need a digital passport photo of yourself. There's an added US$ 5 charge for the e-visa. In my own experience in early 2007, I applied for my visa on a Friday evening, and received my e-visa by email on Monday. On arrival at Siem Reap, I was first in the e-visa line, but there was no officer on duty. When he did show up, it took him several minutes to set up his computer before he could begin processing my passport. There were actually several people without visas who got to baggage claim before me.