Cambodia has two distinct seasons though the year:

Wet Season: Cambodia's wet season comes courtesy of the southwest monsoon which blows from May to October, bringing with it some 75% of Cambodia's annual rainfall. Not surprisingly, wet season is characterized by rain, and in the peak of wet season from July to September it can rain as much as two out of every three days. Rainy days tend to have a few hours of heavy rain rather than being all-day downpours, you will get wet travelling in a Cambodian wet season but the rain makes things different so remember to carry rain coat or umbrella. 

On the upside, monsoonal Cambodia is a beautiful country to travel in. The dust is all gone (turned to mud unfortunately) and the lush greenery of the country returns. Angkor Wat in particular can be stunning in wet season -- the birdlife is far more obvious and the ruins have a unique appearance and feel. Observing Angkor Wat with a lightning storm as a backdrop is an electrifying experience. There are also far fewer travellers in the country, so if you prefer to dodge the crowds, wet season can be a good time to visit.

Regionally, the Cardamom Mountains get the heaviest rain in the country, while the entire coastline gets rough seas and a lot of rain.

Dry Season:  Runs from November to April on the back of the northeast monsoon. Cambodia's dry season runs from October to April, when the dusty northeast monsoon arrives. Blowing like a hair-dryer set to high, the northeast monsoon dries out the country fast. While November and January are quite cool (average temperature: 20-25 °C) by April the weather is scorching and so dry it will take your breath away (average temperatures: 33°C+). Characterized by heat and dust, this season coincides with Cambodia's peak tourist season when travellers arrive in their droves between November and January to take advantage of the lack of rain and the relative cool. By March travelling can be uncomfortable and hot while April can be excruciating.  

As the country dries out, roads get graded and trip times improve dramatically, although they get more dusty. Cambodia's beach strips at Kep, Sihanoukville and Koh Kong bask in brilliant sunshine with clear calm waters, if you're a beach bum, dry season is the great time for you.