Hitler made sadistic attempts at mass murdering the Jewish and other minorities during the World War II to create a perfect community composed only of Aryan race in Germany. Although the Jewish race occupied the majority of the victims, there were also homosexuals, millions of Poles, Soviet PoW, Gypsies, the disabled and religious dissidents like Jehovah's witnesses. There were more than 6 million victims from the Holocaust and various methods were taken to mass murder the undesirable. A few ways of massacre include gas chambers, ovens and experiments with live humans. There are numerous media available nowadays, such as books and films that portray the brutal killing and the aftermath.
Hitler attempted to purify the Aryan race by ridding it of Jews, who were killed as "vermin" and other "inferior" races such as the Polish. Slavs were used as labor or killed without compunction. In addition, the ultimate destruction of Christianity was a Nazi goal and many a Roman Catholic was put in the death camps at Dachau and Auschwitz.
The abuses also included the appaling medical "experimentation" - more akin torture - carried out by Josef Mengele and his associates. Labelled "The Angel of Death", Mengele carried out brutal experiments on inmates in Auschwitz, which included rates of tolerance of extreme cold, poison andd repeated torture. Their defence was that at the time there was no international laws governing medical research.
Let us not forget that it wasn't just Jews who suffered in the Holocaust. Homosexuals, those that tried to hide Jews and conceal them and people with disabilities or other conditions that prevented them from being, from the Nazi's viewpoint, "useful" to society were also sent their to work and die.
Another aspect of the Nazi regime was the ideology of a superior race. They used something call eugenics to justify their ideology. Namely they were the superior race. The rest of the people (particularly the Jews), they sought to kill. There were eventually put into concentration camps and millions were killed. We can say that the Nazi had not understanding of human rights.
The Holocaust specifically refers to the outcome of the so-called "Final Solution": the decision made by Nazi leadership to exterminate the Jewish population of Europe. Yet others faced similar fates, including political dissidents, Roma, Jehovah's Witnesses, people with mental and physical disabilities, and homosexuals.
Are you just asking what happened here? If so, the Holocaust was the killing of millions of people, mainly because of their religion/ethnicity. The main group targeted was Jews. It is generally believed that about 6 million Jews were killed. Other groups were targeted as well, but not in such large numbers.