The Holocast is the mass murder of all the Jews, Muslims, disabled and homosexuals.
Many people didn't know about these murders and were only found out when the allied started pushing into territory once controlled by Germany.
Hitler also made the plan called "The Final Solution". First the Jews were put in labour camps, but now Hitler decided that the only solution was to kill all Jews in Europe.
The Death camps, first Nazi soldiers entered towns and killed all Jewish people but this wasn't fast enough so Hitler build six death camps in Poland. All the Jews were shipped there by train. In the camps some were forced to work and most eventually starved of starvation. But many people were killed in gas chambers. The Allies knew of the camps but only at the end of the war they realised the size and brutality of the camps..