During Medieval Times, there were four main classes of people. Lords and ladies were at the top of the classes. Next were the knights, or fighting class. After that were the groups of religious men, called monks, and women, called nuns, who lived in monasteries and abbies. They gave their life to God and to their religion. The serfs, or poor farmers, were at the bottom of the class system.
A person had to be born into their particular class. If your father was a serf, then you would be a serf. If your father was a lord or noble, then you would be a lord or nobleman. Noble boys and girls who had no hope of inheriting any land or wealth joined the church, and never married. There wasn't anywhere to move up in life.