Cambodian cuisine is an entity. The Khmer foods are typically high in vegetable, rice and fish product. The fish product often is bprahok a fermented fish which is salty and aromatic. It's a reliable source of protein and able to be made by ordinary households and carries the protein resource through the dry season to the prolific new fish stock re-population of the following year. The natural resource Tonle Sap is or was the greatest resource of fresh-water fish in the world. Development and the tragedies of poor management in recent history have taken the edge off that but hopefully this can be reversed. The "greens" range from the abundance of tropical plants typically growing by and on the many waterways of Cambodia. Many of these herbs are aromatic and useful for their flavour characteristics. There is an extensive use of new shoots of shrubs and trees with typical bitterness, the evolution of which in the cuisine is lost in time but is probably associated with need, nutrition and non-toxicity.
Sugar palms are the signature of Cambodia and the juice of the flower parts harvested seasonally. This is a significant proportion of carbohydrate and used extensively for sweetening, wine production and vinegar. Palm sugar vinegar is a particularly "clean" vinegar and can be used where-ever white vinegar is indicated. Palm sugar is easily available and has a soft caramel flavour. Coconut palm sugar and sugar palm sugar are almost identical, if not identical in flavour.
Chinese and Vietnamese influence is strong and many dishes have been imported from these places. The Chinese have had a long time presence in Cambodia and Vietnam-Cambodian borders have altered many times so that the land that is now Mekong Delta and including Saigon was once Cambodia. Thai cuisine is noticeable and once again the borders have played a role. Cambodia has often modified the imported dishes by name and sometimes ingredients but not beyond recognition. The fusion works well.
Northern Cambodian foods have a similarity to Southern Laos cuisine and this is due to border proximity and a shared resource of the Mekong at one of its most bountiful points on the river's course at and near the escarpment which separates the two countries. There is no ocean proximity at this point and the major source of edible protein is fresh water fish which has been abundant.
Original Khmer foods are not so readily seen by travelers. The restaurants serving these are usually not so easy to spot other than the ubiquitous street vendor stalls. Cambodian dishes require a significant preparation time and often involved cooking procedures to bring out the unique characteristics of this very flavoursome cuisine. Cambodia has a unique rice the so called floating rice which has the same height growing rate as the filling of the lakes during the wet season. There is some speculation that this may have been the original rice. It can be eaten today but I have never done so. On enquiring with various contacts there seems to be a consensus that contemporary rices are superior in flavour. Sticky rice is also used but not so widely; mostly in dessert dishes.
Interestingly there is an underlying shyness with regard to promoting Khmer food. This has been noticed in western countries also and now just changing. Typically Khmer restaurant owners would label their establishment as Thai food and introduce selected dishes as Khmer within the menu generally of the Surin Thai-Khmer tradition.
Khmer cuisine is now re-emerging. Interestingly it is the rural or peasant cuisine that is most noticeable and little of the Royal Cuisine. Khmer cuisine will be more and more noticeable in time for three reasons. Travel, training and confidence.