'We believe that Jesus Christ, the son of God, became human. And that when he came among us, he was and is our true and eternal high priest. He is also the prophet of God and he is truly the shepherd king or ruler of God's people." (Msgr. Zenz from "The Sacrament of Holy Orders" video.)
The sacrament of Holy Orders is the activity by which a man commits his life to serve the Church.
Holy orders is a three-fold sacrament in that it includes ordination to the diaconate, the priesthood and to the office of Bishop.
The deacon is ordained for service and can perform marriages, baptisms, assist at the altar during Mass and performs other rituals in the Church.
When a man is ordained to the priesthood he is given the responsibility and the power to offer Mass (the Eucharist), forgive sins, administer the other sacraments and care for the spiritual lives of the people he serves.
When a Bishop is ordained he accepts the responsibility to administer his diocese and has the personal jurisdiction as a successor of the apostles over the people in his diocese.