不如别人的人：people who are not as good as others when compared . 【she's beautiful and smart, i'm not as good as her.她漂亮又聪明，我不如她。】 低下的人：it's about rankings of people, it means the people who have an inferior status 【In the Middle Ages,a serf had a very inferior status in life.中世纪时，农奴地位低下。】 自卑：self-contempt , a sense of inferiority 【I always felt inferior to others before. 我以前总是觉得很自卑。】 较差的：the worse one 【The food at that restaurant is inferior. 这家餐馆的食物较差。】 You are right about 下级，低等的，差的，次的。