The Holocaust is a pretty far reaching event.
When Nazi Germany rose, they followed a policy of eugenics - the creation of a "perfect" race. Their idea of the perfect race was a society of Blond haired, blue eyed German "Aryan" people. Shortly after Hitler took power forced sterilizations began to occur against disabled people. Eventually many were gassed in mobile killing vans, otherwise known as "Einsatzgruppen"... this would become something far worse in the near future.
Hitler also targeted Jewish people. He blamed them for Germany's loss of World War I, and saw them as the "inferior race" He incited riots against them, and many Jewish places of worship and residence were destroyed in what became known as "Kristalnacht"
Eventually things went from bad to worse. Jewish people began being put in labor camps, or organized into ghettos... small, closed off sections of cities meant only for Jewish people. It was more or less a prison. World War II began when Germany invaded Poland. As Germany marched east, the Einsatzgruppen began indiscriminantly killing Jewish people (even eastern europeans of non-jewish origins) along their way, at least those who were not rounded up and sent to the camps. Read a book called "Ordinary Men" to learn more about that.
Jewish people that were rounded up met a fate far worse than the Einsatzgruppen in most cases. They were sent to the camps, and worked to death, starved, or gassed. Some underwent hideous medical experiments at the hands of Joseph Mengele in Auschwitz, one of the more grusom of the camps.
Six million Jews died in the Holocaust. Six million OTHER people including Gays, Gypsies, Disabled people, communists, and many other "undesirable groups" were killed. A total of 12 million people were murdered at the hands of the Nazi regime... and that's only in the camps, discounting war fatalities... for example 20 million people died on the Soviet front.
Many say the Holocaust is unique. We still see unimaginable genocide in our world today, but nothing quite to the extent of the Holocaust. For the first and only time in history you had a modern industrialized first-world nation using technology, government resources, and science combined to systematically, mathematically, and brutally murder an entire race of people. It would be the equivilant of the United States doing such a thing today.
Others say the Holocaust is not unique. Joseph Stalin, the leader of the Soviet Union durring World War II once said "If you kill one, it's a tragedy. If you kill 10 million, it's a statistic." A death at the hands of a brutal government is a horrible. This still happens today.
Even others say the Holocaust never happened. They think it's a Jewish conspiracy. Those people usually base these oppinions on racism, political beliefs, or ignorance... in my oppinion at least. Make up your own mind, but I will say you will have an extremely tough time finding any reputable historian that denies that the holocaust occurs.
Anyways it's a horrible thing. The links others have provided to your answer offer great insight into the essence of what the Holocaust was. For further information I suggest watching a few movies and reading a few books.
Escape from Sobibor
Diary of Anne Frank
A book I can't remember, but the author is Olga Lengyel.. google that you'll find it.