The Holocaust (from Greek words meaning "whole-burnt") was the genocide of approximately six million European Jews during World War II by the Nazis and their leader Adolf Hitler. The "prisons" were concentration camps, in which Jews were used for slave labor and then exterminated in gas chambers.
The Nazis also systematically murdered ethnic Poles, disabled people, homosexuals, and people of opposing political or religious views. Including these groups, the total number of Holocaust victims was between 11 million and 17 million people.
Miss Gates condemns Hitler for his prejudice against the Jews whilst she herself is prejudiced against African-Americans.