The Catholic Church teaches that when a man participates in priesthood, he participates in the priesthood of Christ Himself. All men who, through the Sacrament of Holy Orders, have become priests participate in Christ's priesthood; they act in persona Christi Capitis, in the person of Christ, the Head of His Body, the Church.
Unlike usage in English, "the Latin words sacerdos and sacerdotium are used to refer in general to the ministerial priesthood shared by bishops and presbyters. The words presbyter, presbyterium and presbyteratus refer to priests in the English use of the word or presbyters." According to the Annuario Pontificio 2016, as of December 31, 2014, there were 415,792 Catholic priests worldwide, including both diocesan priests and priests in the religious orders. A priest of the regular clergy is commonly addressed with the title "Father" (abbreviated Fr., in the Catholic and some other Christian churches).
The state of consecrated life or monasticism is a separate, third distinct vocational state from the clergy and the laity. As an overview, there are the members of the laity—who are married or unmarried, and the clergy—the bishops, priests, and deacons. Deacons are male and usually belong to the diocesan clergy, but, unlike almost all Latin-rite (Western Catholic) priests and all bishops from Eastern or Western Catholicism, they may marry as laymen before their ordination as clergy.
Members of institutes of consecrated life, or monks, can be either clergy or non-ordained members of the religious order (male or female non-ordained religious are not to be considered laypersons in the strict sense—they take certain vows and are not free to marry once they have made solemn profession of vows; all female religious are non-ordained, they may be sisters living to some degree of activity in a communal state, or nuns living in cloister or some other type of isolation). The male members of religious orders, whether living in monastic communities or cloistered in isolation, and who are ordained priests or deacons constitute what is called the religious or regular clergy, distinct from the diocesan or secular clergy. Those ordained priests or deacons who are not members of some sort of religious order (secular priests) most often serve as clergy to a specific church or in an office of a specific diocese or in Rome.
The New Testament depicts Jesus as the "great high priest" of the New Covenant who, instead of offering the ritual animal sacrifices prescribed by the Jewish Law, offers himself on the cross as the true and perfect sacrifice. The Catholic priesthood is a participation in this priesthood of Christ, and therefore traces its origins to Jesus Christ himself. Thus, the New Testament says that as high priest, Jesus has made the Church "a kingdom of priests for his God and Father." All who are baptized are given a share in the priesthood of Christ; that is, they are conformed to Christ and made capable of offering true worship and praise to God as Christians. "The whole community of believers is, as such, priestly."
The ministerial priesthood of Catholic priests and bishops—what most people think of as "the Catholic priesthood"—has a distinct history. This ministerial priesthood is at the service of the priesthood of all believers and involves the direct consecration of a man to Christ through the sacrament of orders, so that he can act in the person of Christ for the sake of the Christian faithful, above all in dispensing the sacraments. It is understood to have begun at the Last Supper, when JesusChrist instituted the Eucharist in the presence of the Twelve Apostles, commanding them to "do this in memory of me."
The Catholic priesthood, therefore, is a share in the priesthood of Christ and traces its historical origins to the Twelve Apostles appointed by Christ. Those apostles in turn selected other men to succeed them as the bishops (episkopoi, Greek for "overseers") of the Christian communities, with whom were associated presbyters (presbyteroi, Greek for "elders") and deacons (diakonoi, Greek for "servants"). As communities multiplied and grew in size, the bishops appointed more and more presbyters to preside at the Eucharist in place of the bishop in the multiple communities in each region. The diaconate evolved as the liturgical assistants of the bishop and his delegate for the administration of Church funds and programmes for the poor. Today, the rank of "presbyter" is typically what one thinks of as a priest, although technically both a bishop and a presbyter are "priests" in the sense that they share in Christ's ministerial priesthood and offer sacrifice to God in the person of Christ.