The word ordo (order, in Latin) designated an established civil body or corporation, and ordinatio meant legal incorporation into an ordo. The 3 degrees of Holy Orders represent ordines.
The Catholic church sees its priesthood as both a reflection of the ancient temple priesthood of the Jews and the person of Christ. The liturgy of ordination recalls the Old Testament priesthood and the priesthood of Christ. In the words of St. Thomas Aquinas, "Christ is the source of all priesthood: the priest of the old law was a prefiguration of Christ, and the priest of the new law acts in the person of Christ" [Summa Theologica III, 22, 4c].
The arrangement given above, "bishops, priests, and deacons" is in the reverse order of ordination. Typically in the last year of seminary[?] training a man will be ordained to the diaconate, called in recent times the "transitional diaconate" to distinguish men bound for priesthood from those who have entered the "permanent diaconate" and do not intend to seek further ordination. Deacons, whether transitional or permanent, are licensed to preach sermons, to perform baptisms, and to witness marriages, but to perform no other sacraments. They may assist at the Eucharist or the Mass, but are not the ministers of the Eucharist.
After at least six months or more as a transitional deacon a man will be ordained to the priesthood. Priests are able to preach, perform baptisms, witness marriages, hear confessions and give absolutions, and celebrate the Eucharist or the Mass.
Bishops are chosen from among the priests, and are the leaders of territorial units called dioceses. Only bishops can validly administer the sacrament of holy orders. In Latin-rite Catholic churches, only bishops may lawfully administer the sacrament of confirmation, but if an ordinary priest administers that sacrament illegally, it is nonetheless considered valid, so that the person confirmed cannot be confirmed again, by a bishop or otherwise. In Eastern-rite Catholic churches, confirmation is done by parish priests via the rite of chrismation, and is usually administered to neonates immediately after their baptism.
Roman Catholics recognize the validity of holy orders administered in Eastern Orthodox and Oriental Orthodox churches because those churches have maintained the apostolic succession of bishops, i.e., their bishops claim to be in a line of succession dating back to the Apostles, just as Catholic bishops do. Consequently, if a priest of one of those eastern churches converts to Catholicism, he is automatically a Catholic priest. Eastern Orthodox bishops can, and normally do, grant recognition to the holy orders of converts who were earlier ordained in the Catholic church; that is part of the policy called church economy. Anglican churches, unlike most Protestant churches, maintain the succession, their bishops being successors of English bishops who converted to Protestantism in the 16th century. A controversy in the Catholic church over the question of whether Anglican holy orders are valid was dogmatically settled by Pope Leo XIII in 1896, who wrote that Anglican orders lack validity because the rite by which priests are ordained is not correctly performed. Eastern Orthodox bishops generally grant "economy" when Anglican priests convert to Orthodoxy. Catholics do not recognize ordination of ministers in Protestant churches that do not maintain the apostolic succesion.
Married men may be ordained to the diaconate as Permanent Deacons, but in the Latin Rite of the Catholic Church may not be ordained to the priesthood. In the Eastern Rites of the Catholic Church and in the Eastern Orthodox Church married deacons may be ordained priests, but may not become bishops. Bishops in the Eastern Rites and the Eastern Orthodox churches are drawn only from among monks, who have taken a vow of celibacy.
There are cases of permanent deacons who, left widowed by the death of a wife, have been ordained to the priesthood. There have been some situations in which men previously married and ordained to the priesthood in the Anglican Church have been admitted to the Catholic priesthood and allowed to function much as an Eastern Rite priest but in a Latin Rite setting.
There is a difference between chastity and celibacy. Celibacy is the state of not being married, so a vow of celibacy is a promise not to enter into marriage but instead to consecrate one's life to service (in other words, "married to God"). Chastity, a virtue expected of all Christians, is the state of sexual purity; for a vowed celibate, or for the single person, chastity means the avoidance of sex. For the married person, chastity means the practice of sex only with the spouse, and even carries the expectation of intercourse with the spouse preferably at the sole scope of reproduction.
Not all priests have lived up to these ideals: see Catholic priests' sex abuse scandal.
See also: clergy