Holy Orders is the sacrament through which the messianic mission of Christ continues in His Church until the end of time.
The three degrees of this one sacrament (episcopate, presbyterate, and diaconate) are a participation in the apostolic offices of teaching, sanctifying, and governing given by the Lord Jesus to the Twelve. In Roman law, the word "order" designated a group or civil body within society. "Ordination" simply means incorporation into an "order."
Sacred Scripture describes to us the three offices of ministry proper to the New Covenant, and each of these offices constitutes a single such "order" in the Church: the Order of Bishops, the Order of Priests (or Presbyters), and the Order of Deacons. A baptized man is ordained into one of these three Orders by a prayer of consecration and the laying on of hands by a true bishop in apostolic succession, and this liturgical action of Christ and the Church confers on the one ordained the sacred power to preach the Word of God and administer the other sacraments, according to the station of each Order.