The Holocaust was the apex of centuries of anti-Semitism brewing in Europe. Every so often it erupts, and you have Jews massacred throughout European history. In my opinion, the Holocaust was the climax of that chain of incidents.
The Holocaust was the "final solution" to ridding Germany of its Jewish population. Ever since the Nazis gained power in 1933, they drew up legislation to marginalize not only Jews, but Gypsies, communists, Jehovah's Witnesses, Catholics, and homosexuals. First, they were to register with the government as Jews, communists, or whatever they were. Typically thereafter, their businesses were looted, their property was confiscated, and they were forced to live in squalor in various ghettos, most notoriously in Warsaw and Krakow.
If theft and segregation weren't enough, these groups were then herded into work camps, treated as slaves until they dropped dead from exhaustion or killed for trumped-up charges. It was slow and agonizing, and it helped prop up the German economy at a time when every German male with hair on his chest went to the military.
As World War II trugged on, and Hitler saw defeat as a possibility, he was no longer satisfied with the slow death of the work camps. That's when the real genocide started kicking in, where you hear the stories of people shoved in ovens and burned alive, where hundreds were rounded up and gassed to death, where Jews were killed indiscriminantly. This became the prime focus of the Thord Reich in the later years of the war, especially after D-Day, when you had the American-British-Canadian forces squeezing Germany from the west, and the Soviet Union from the east. The closer Germany came to defeat, the more urgent the call was to exterminate everyone in the camps. It was such a high priority for Hitler that, instead of using his oil supply to keep his military functioning, he used it to keep the trains running that shipped Jews to the camps, and while German tanks stalled, German trains continued.
The agreed-on figure is six million Jews dying in the Holocaust -- the"burning" -- but it should never be forgotten that many other people died as a result of Hitler's final solution. As many as a million Gypsies were murdered alongside their Jewish neighbors. Hundreds of thousands of communists were killed as enemies of Hitler's fascist regime. Many more thousands of Jehovah's Witnesses and Catholics also perished. It's possible as many as eight to ten million people were systematically murdered by the Third Reich in what was call the Holocaust.