The main language of Cambodia is Khmer. It is in the same language family as Vietnamese and Lao.
Cambodia uses the riel, officially. 4,000 riel is the equivalent of one U.S. dollar. However, a lot of stores and restaurants will accept U.S. dollars and only hand you riel when you need small change. My friends and I found that we mainly used dollars while in Cambodia.
A regular sized bottle of water costs about 1,000 riel or about one quarter in U.S. dollars. It was so hot that I would often buy a larger bottle that could cost 3,000 to 4,000 riel.
I didn't know much about Cambodian food before this trip, yet I found that Khmer cuisine reminded me of Thai food. One of my favorite meals was a red curry. I had it at a cafe near my hostel. It was not as spicy as I expected and it included a lot of vegetables and bamboo shoots. Fresh fruit shakes are also very common and you can buy one for only one dollar!