most of them were jewish . because they were jewish (its a religion and also a nationality as in all from the same country ) . and then also intelligent people and russian people and handicapped people and probably old people . the people didnt want them around and the people voted for adolf hitler to get rid of them .
Hitler considered that there were two races in the world - the Master race ie Aryans(Germans) and Non Aryans, .So he considered himself to have entire authority of killing rebels, he also considered Jews to be the ones responsible for all the defamatory appearence of Jews....so he murdered them brutally......................during the holocaust he would gather large no of Jews in concentration camps and tortured them. he leaked mustard gas in such chambers full of Jews and they were instantly poisoned and died
if you want to know about the terrible conditions during those days read THE DIARY OF ANNE FRANK
Holocaust is a term pertaining to Jews. However Gypsies were also victims (gypsies were in fact first to live in ghettos prior to WW2), The Polish intellectuals were slaughtered en masses, Many Poles, Russians, French died in Concentration camps. Reasons were crazy Nazi way of thinking and living space for Germans