Answer: Spiritual life
Serfs were laborers or peasant of the middle ages. They provide manual labor on the lands of their landlords. They paid taxes and were responsible for everything that involves the lords' property.
Clergy in the Middle Ages was very influential in the society. In the Roman Catholic Church, they refer to ordained priests or bishops. They were exempted from paying taxes, unlike the serfs. They give services to their parishioners. They provide spiritual satisfaction and care to serfs.