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 1536).
"This is why I left you in Crete, that you might amend what was defective and appoint presbyterous [Priests] in each community." - Titus 1:5
"Take care to do all things in harmony with God, with the bishop presiding in the place of God, and with the presbyters [Priests] in the place of the council of the apostles, and with the deacons, who are most dear to me, entrusted with the business of Jesus Christ, who was with the Father from the beginning and is at last made manifest" (St. Ignatius of Antioch, Letter to the Magnesians 6:1[110 A.D.])