Sacrament of Holy Orders

“Holy Orders is the sacrament through which the mission entrusted by Christ to his apostles continues to be exercised in the Church until the end of time: thus it is the sacrament of apostolic ministry. It includes three degrees: episcopate, presbyterate, and diaconate.”
– CCC 1601

THE PRIESTHOOD OF THE OLD COVENANT

The chosen people was constituted by God as “a kingdom of priests and a holy nation.” But within the people of Israel, God chose one of the twelve tribes, that of Levi, and set it apart for liturgical service; God himself is its inheritance. A special rite consecrated the beginnings of the priesthood of the Old Covenant, the priests are “appointed to act on behalf of men in relation to God, to offer gifts and sacrifices for sins.”
CCC 1539

THE ONE PRIESTHOOD OF CHRIST

Everything that the priesthood of the Old Covenant prefigured finds its fulfillment in Christ Jesus, the “one mediator between God and men.” The Christian tradition considers Melchizedek, “priest of God Most High,” as a prefiguration of the priesthood of Christ, the unique “high priest after the order of Melchizedek”; “holy, blameless, unstained,” “by a single offering he has perfected for all time those who are sanctified,” that is, by the unique sacrifice of the cross.

The redemptive sacrifice of Christ is unique, accomplished once for all; yet it is made present in the Eucharistic sacrifice of the Church. the same is true of the one priesthood of Christ; it is made present through the ministerial priesthood without diminishing the uniqueness of Christ’s priesthood: “Only Christ is the true priest, the others being only his ministers.
CCC 1544-1545

– Adapted from Catechism of Catholic Church, more details here.