The "Holocaust" refers to the period from January 30, 1933, when Hitler became Chancellor of Germany, to May 8, 1945 (V-E Day), the end of the war in Europe.
The Holocaust was, as some people thought, a genocide of about 6 million Jews as well as political opponents, Roma Gypsies, Blacks, and more. It did not began as the Holocaust yet, there was different stages that occurred that built up to the "Final Solution". It began with laws that had required Jews to be re- moved from the general population. They, including Romani, were placed into ghettos, which were overcrowded and filthy areas of cities that were to be used as holding areas
Ghettos
During this time, concentration camps were also established, and Jews and Romani were pulled from the ghettos and placed into the concentration camps, where they were forced into slave labor until disease, starvation, or exhaustion killed them. As Nazi's continue to take over Europe, new ghettos and concentration camps were created. Death squads began to execute Jews and others in mass shootings, burying them in mass graves throughout the continent.
Later during WWII, Nazis set the Final Solution into motion. Concentration camps were becoming extermination camps. The main purpose of them was to execute, kill, as many people as they could in as a efficient a manner as possible. Millions died in those camps. It was a sad, terrible, tragic time.