The Khmer Rouge killed an estimated 2 to 3 million Cambodians via torture, execution, over-work or starvation during its four-year reign of terror. (This was 1/4 or 1/5 of the total population.) They sought to cleanse Cambodia of capitalists and intellectuals, and to impose a new social structure based entirely on collective agriculture.
Pol Pot's murderous regime was forced out of power by a Vietnamese invasion in 1979, but the Khmer Rouge fought on as a guerrilla army from the jungles of western Cambodia until 1999.
Today, some of the Khmer Rouge leaders are being tried for genocide and crimes against humanity. Pol Pot himself died in 1998, before he could face trial.
The term "Khmer Rouge" comes from Khmer, which is the name for the Cambodian people, plus rouge, which is French for "red" - that is to say, Communist.
Even thirty years later, the people of Cambodia have not fully recovered from the horrors of the Khmer Rouge's murderous reign.