Many websites give poor climate information as they tend to merge several countries in Southeast Asia in their weather forecast. In Cambodia, there is no monsoon with continuous rain, but rather showers lasting from 1 to 2 hours in the late afternoon or at night time. The climate is tropical - hot all year round. 

Temperature 

From November to March, temperatures are pleasant, varying from 18°C at night in mountainous areas to 33°C.
April-May is a warmer season, from 33°C reaching almost 40°C in May. Mid-May,  light rains start lowering temperatures between 26°C and 32°C depending on the place.
July-August is also a good season to visit, with temperatures from 26°C to 32°C thanks to the humidity.
September-October is still a good time to come to Cambodia with some light rain bringing some coolness compared to May.
Monsoon 
Cambodia's monsoon is very different from the ones in Thailand or Burma. In July-August, it will rain a maximum of 1 to 2 hours, usually in the late afternoon and a few times a week. It helps to bring some freshness and to maintain pleasant temperatures during the summer months.