Of course there were many things that characterized the Medieval Period. The two that might distinguish the period the most are the dominance of the Catholic Church and the institution of feudalism.
Throughout the medieval period, nearly everyone in Western Europe was Catholic. This was before the Protestant Reformation of the sixteenth Century and the split of many factions of the Church. There were notable populations of Jews scattered around the continent, Muslims in Spain, and Eastern Orthodox Christians in the east. However, the Catholic Church held dominance over most of Europe and Europeans.
The pope often held more power than even monarchs. A king who disobeyed the wishes of the Church often did so at great risk. For instance, when King Henry IV of the Holy Roman Empire disagreed with the pope he found himself in a dire situation. When an argument arose over who had the power to appoint local clergy, Pope Gregory VII demonstrated his authority by excommunicating the king and announcing that Henry's subjects were no longer under his rule. King Henry's excommunication was only revoked after he begged the pope for forgiveness.
The Church held sway over kings and peasants alike. It was a truly religious time. Laws of states were usually in accordance with the Church's biblical interpretations. The belief in heaven, hell, and salvation was very real. The Church used its influence during the medieval period to become the richest and most powerful institution in Europe.
In addition to the role of the Church, the medieval period was also dominated by feudalism. Feudalism was the hierarchy structure in which land and power was divided and shared. Feudalism is often described as a pyramid. The monarch was at the top. He (and sometimes she) owned all the land in the kingdom. To manage this land, the king divided parcels of the kingdom, or fiefs, among his nobles. In return for the land and the privilege of noble rank, the nobles payed taxes to the monarch and supported him politically and militarily. These nobles then divided up their fiefs among lower-ranking nobles and so forth.
Feudalism placed a strict hierarchy on medieval society. Monarchs owned the land, but it was managed by nobles. Knights, who were usually lower-ranking nobles promised to serve the nobles and king on whose land they managed. To work the land, peasants and serfs toiled in the fields. They paid taxes, usually in the form of a percentage of the harvest, to nobles. These nobles, in turn, allowed the peasants to live and work on their land and protected them during times of war.