The Sacrament through which the mission entrusted by Jesus to His Apostles continues to be exercised in the Church to the end of time. Saint Thomas Aquinas makes the important point that only Christ is the true priest, the others serving as His ministers (Hebrews 8:4). Bishops are the successors of the Apostles, and priests and deacons are his assistants in rendering service. Men are ordained to the priesthood in the Catholic and Orthodox Churches, as the Sacrament confers upon the priest the character to act in the person of Christ - in persona Christi.
Holy Orders is the Sacrament of Apostolic ministry. The rite consists of the Bishop's imposition of hands on the head of the priest-candidate with the consecrating prayer asking God for the outpouring of the Holy Spirit for the gifts of the ministry. There are three dimensions to ministry, that of Bishop, Priesthood, and the Diaconate. See Matthew 16:18-19, John 21:15-17, Romans 10:14-15, 2 Timothy 1:6, and Titus 1:5.