The priestly caste is a social group responsible for officiating over sacrifices and leading prayers or other religious functions, particularly in nomadic and tribal societies.
In some cases, as with the Brahmins of Vedic India and the Kohanim and Levites of ancient Israel, the caste was a hereditary one, with a person's position as a priest depending on his biological descent. Zoroastrianism also has a hereditary priesthood, as does Alevism, Yezidism and Yarsanism. In Sufism, the spiritual guide is also often a hereditary leader, while the Sayyids of India, who claim descent from the Islamic Prophet Muhammad, have been described as a priestly caste.
In other cases, as with the Druids of the Celtic world and the shamans of ancient Eurasian nomads, the position within the caste may have depended more upon apprenticeship; the exact nature of the "caste" in these cases is difficult to ascertain due to our lack of primary sources.