In the Middle Ages, land=money so the king, who owns ALL the land is pretty much the highest person in the kingdom. Under them are the lords and ladies, the friends of the king. Since many kings had to much land to govern on, so they divide the land up to the lords and ladies to govern. The people who received the land were called vassals. In trade for this land, the vassal must give full loyalty and military service to the king.
Under the nobles are the knights, or lesser lords. In a very large kingdom, the king will give very large plots to the nobles, resulting in the nobles not able to control the land their given, so they give it to knights, who become vassals. For this trade they must promise to be a part of the king's military if an enemy comes. And finally, underneath them is the pheasants, or the serfs. They have no freedom at all and they are part of the land so if the vassal sells the land, they go along with them. They live on a small portion of the vassal's land. In return, they must pay taxes and give a share of crops to the vassal. This guarantees them protection from the barbaric tribes such as the Vikings.