Peasants are people who farm or do labor to make a living. They don't work for the king, so they wont be protected by him if they are under attack. Serfs are people who farm or work for the king. In return, they would be protected if they they are under attack.
Knight are people who fought for the king. They rode on horseback and had very heavy amrour and weapons. However, only very rich nobles could own their own knight.
Kings were on the top of the feudalism pyramid. They were very rich and owned the land. Their position was passed down to them, so they didn't have to earn it. They were very respected and had luxorius lives.
Churches were very important to them. Churches and church officials were almost as important to them as the king. The most important church official was the pope. Christianity united them after they broke into 6 different kingdoms.
Vassals played a big role in the feudal system. They paid taxes on land and pledged military services. The lords had to pledge to protect them in return for their service.
Charlemagne was a ruler who united the kingdoms under one rule. His 2 main goals were to unite the broken kingdoms and create new laws. He succeeded in both. He also spread Christianity, protected his subjects and organized the empire.