Being ripped away from your families on the ramp as you exited the train.
Being shaved and tattooed against your will.
Lack of food. At times they were living on only 500 calories a day.
Forced labor. They worked very hard from dawn to dusk and given the hard work and lack of food, their health began to deteriorate.
So many people in one place caused rapid spread of diseases like typhus and diphtheria and once you contracted those diseases you would either be deemed unfit to work and sent to the gas chambers OR you died naturally (most went to the gas chamber).
Being forced to do things like burn bodies, bury bodies, count gold teeth, sort the clothes of the dead etc etc., all jobs that were terrifying as you knew you would yourself be burned one day.
Having open cess pits in the middle of the barracks which made disease rampant and putrid.
Being forced to stand for roll call in the freezing cold and snow without coats or in the blazing sun.
Seeing people shot or hung for minor "offenses" in front of you.
Lack of any emotion other than terror which eventually becomes the only emotion you know. My grandfather said "Once hopelessness crept in and attached itself to the terror in your mind, you ran towards that fence" (which meant you either were electrocuted or shot by a nazi on the guard tower but either way it was suicide).
Having no prospect of a future or a clue as to when your plight might end.
Enduring beatings and whippings at the hands of the nazi's or more often the hands of some criminal kapo who was over you.
Never being clean.
Always being thirsty.