Short answers, pulled from the story.
Pol Pot, born Saloth Sar, became the architect of a regime that killed millions. He served as Prime Minister of Democratic Kampuchea after the state was officially established on the 5th of January 1976.
The Khmer Rouge captured Phnom Penh on the 17th of April 1975, ending the Lon Nol regime. This victory came after years of insurgency supported by Chinese military aid and Vietnamese shelter.
Historian Ben Kiernan estimates that between 1.671 million and 1.871 million Cambodians died due to Khmer Rouge policy. This represented between 21% and 24% of Cambodia's 1975 population.
China provided more than 15,000 military advisers to Democratic Kampuchea after 1975. At least 90% of all foreign aid came from Beijing, including US$1 billion in interest-free economic and military assistance in 1975 alone.
Vietnamese forces overthrew Democratic Kampuchea on the 7th of January 1979, destroying most of its army. The Khmer Rouge fled to Thailand where their government was seen as a buffer against Communist Party of Vietnam expansion.