The Holocaust (Also known as Ha-Shoah) refers to a series of programs of National Socialist Germany during the Second World War involving the mass killings of at least eleven million "undesirables", most notably six million Jews, in accordance with Nazi racist ideology.
A small number of White Supremacists and anti-Semites claim that the Holocaust never happened as a means of keeping face and trying to convince others that they are not a bunch of small-minded, hate-filled barbarians. Such claims fly in the face of mountains of evidence including documents, sites, mass graves, the major decline of the Jewish population (particularly the near complete destruction of Poland's Jewish population of three million) in Europe, and so on. The near extermination of the Polish and east European Roma (aka "Gypsy") population is just one more nail in the denier's coffin.
Most of the deniers tend to bank on the assumption that the Holocaust only involved the Jews, ignoring the numerous other groups that were also targeted, including, Gypsies, homosexuals, the handicapped, Jehovah's Witnesses, Mormons, and many others -- hoping to capitalize on anti-Semite agendas. While the 6 million or so Jews do make up the majority of the people killed, it was only a part of the 11-13 million casualties that are estimated as a part of the Nazi's final solution.