Clergy is the generic term used to describe the formal religious leadership within a given religion. The term comes from Greek κλήρος - kleros, "a lot", "that which is assigned by lot" (allotment) or metaphorically, "heritage"^[1]. Depending on the religion, clergy usually take care of the ritual aspects of the religious life, teach or otherwise help in spreading the religion's doctrine and practices. They often deal with life-cycle events such as childbirth, baptism, circumcision, coming of age ceremonies, marriage, and death.

A priesthood is a body of priests, shamans, or oracles who have special religious authority or function. The term priest is derived from Latin presbyter, but is often used in the sense of sacerdos in particular, i.e., for clergy performing ritual within the sphere of the sacred or numinous (ta hiera) communicating with the gods on behalf of the community.

There is a significant difference between clergy and theologians; clergy have the above-mentioned duties while theologians are scholars of religion and theology, and are not necessarily clergy. A lay-person can be a theologian.

In Christianity there is a wide range of formal and informal clergy positions, including deacons, priests, bishops, and ministers. In Islam, religious leaders are usually known as imams or ayatollahs.