The Nazis started off making Jews dig their own graves and shooting them but it took a lot of time and some number cruncher worked out it was too costly as bullets were an expensive commodity. Local Germans and Austrians used to go and watch Jews killed this way, almost as some sort of spectator sport. The concentration camps were used to kill more efficiently and cheaply. Exact numbers aren't known but 6 million is the estimated number of Jews killed by the Nazis by all methods.
You don't have to go that far back for similar atrocities. Khmer Rouge in the 70's & 80's (estimated 2 million killed), Rwanda in the 90's (up to a million) etc, but the Holocaust deniers can't be bothered with these as the victims were Africans and Asians, somehow not worth the attention. The apologists and deniers find the whole cult of personality vibe around Hitler to be far more interesting and there is more than a suggestion that somehow the Jews had it coming.
