The priestly caste is a social group responsible for officiating over sacrifice

Sacrifice

Sacrifice is the offering of food, objects or the lives of animals or people to God or the gods as an act of propitiation or worship.While sacrifice often implies ritual killing, the term offering can be used for bloodless sacrifices of cereal food or artifacts...

s and leading prayer

Prayer

Prayer is a form of religious practice that seeks to activate a volitional rapport to a deity through deliberate practice. Prayer may be either individual or communal and take place in public or in private. It may involve the use of words or song. When language is used, prayer may take the form of...

s or other religious functions, particularly in nomad

Nomad

Nomadic people , commonly known as itinerants in modern-day contexts, are communities of people who move from one place to another, rather than settling permanently in one location. There are an estimated 30-40 million nomads in the world. Many cultures have traditionally been nomadic, but...

ic and tribal

Tribe

A tribe, viewed historically or developmentally, consists of a social group existing before the development of, or outside of, states.Many anthropologists use the term tribal society to refer to societies organized largely on the basis of kinship, especially corporate descent groups .Some theorists...

 societies.

In some cases, as with the Brahmins of Vedic India

India

India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

 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. In other cases, as with the Druids of the Celtic world and the shamans of ancient Eurasia

Eurasia

Eurasia is a continent or supercontinent comprising the traditional continents of Europe and Asia ; covering about 52,990,000 km2 or about 10.6% of the Earth's surface located primarily in the eastern and northern hemispheres...

n 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.