Most Cambodians are descendants of the Angkor Empire that extended over much of Southeast Asia and was very powerful between the 10th and 13th centuries. Around that time, there were major wars with the Thai and Cham which weakened the once-strong empire. This started a long period of decline and in 1863 the king of Cambodia put the country under the protection of France. In World War II, the Japanese occupied Cambodia, but after the war Cambodia gained full independence from France.
After a five-year struggle, in April 1975 the Communist Khmer Rouge forces led by Pol Pot captured the capital city, Phnom Penh. As the brutal Khmer Rouge tightened their grip on Cambodia, they evacuated all cities and towns and sent the people into the country to be re-trained in the ways of communism. During this time of genocide at least 1.5 million Cambodians died from execution, forced hardships, or starvation during the Khmer Rouge regime.
Neighboring Vietnam drove out the Khmer Rouge, but it was nearly 17 years after the Paris Peace Accords (which ended the war with Vietnam) before a relatively peaceful national election took place in 2008. Today, Cambodia is governed by a multi-party democracy under a constitutional monarchy. So, yes, they still have a King AND a Prime Minister.