For questions about The Holocaust, the genocide committed by Nazi Germany under the direction of Adolf Hitler. For any other genocide, use the [genocide] tag. This is a sensitive and heavily-trolled topic (many questions are from holocaust deniers trying to get people to agree with them) so questions on this subject are held to an exceptionally strict standard.
The Holocaust was the systematic persecution and genocide by Nazi Germany and their collaborators of millions of Jews during the 1930s and early 1940s. Many people also include the same regime's systematic killing of other groups--including Romanis, Poles, Soviet POWs, the disabled, homosexuals, communists, Jehovah's Witnesses, and other groups--under the same label.