Short answers, pulled from the story.
The Khmer Rouge was the popular name for members of the Communist Party of Kampuchea, which ruled Cambodia as Democratic Kampuchea between 1975 and 1979. It was led by Pol Pot, along with Nuon Chea, Ieng Sary, Son Sen and Khieu Samphan.
The most widely accepted estimates range from 1.5 million to 2 million deaths, roughly 25 percent of Cambodia's population. Deaths came from execution, forced labor, starvation, and famine caused by the regime's agricultural policies.
King Norodom Sihanouk coined the French phrase Khmers rouges, meaning red Khmers, in the 1960s to describe Cambodia's communist-led dissidents. English speakers later adopted the corrupted form Khmer Rouge.
After capturing Phnom Penh on the 17th of April 1975, the Khmer Rouge forcibly evacuated city residents to create a self-sufficient, classless society based on collective agriculture. Residents were told the move was temporary and due to the threat of American bombing, but they were sent on long marches that killed thousands.
China was the main patron, with at least 90 percent of the foreign aid to the Khmer Rouge coming from China and approval from Chairman Mao Zedong. After 1979 a group including China, the United States and Thailand backed the Khmer Rouge coalition to retain Cambodia's United Nations seat against the Soviet Union and Vietnam.
Yes. On the 7th of August 2014 the United Nations-backed Extraordinary Chambers in the Courts of Cambodia jailed Nuon Chea and Khieu Samphan for life, and in November 2018 both were convicted of genocide against the Vietnamese, with Nuon Chea also found guilty of genocide against the Cham. Ieng Sary was arrested in 2007 but died before a verdict.