Read more about the Sacrament of Holy Orders in the Catechism of the Catholic Church.
The Sacrament of Holy Orders is one of two ‘Sacraments at the Service of Communion.’ The Catechism of the Catholic Church describes Holy Orders as ‘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)
It is a ‘Sacrament of Communion’ because it exists to give structure to the Church, to call the members of the Church to a deeper and life-long conversion of heart to Christ, and to be a sign of the age to come ‘where there is neither marrying nor giving in marriage. (cf. Luke 20:35)
The Sacrament of Holy Orders has a three-fold structure the flows from Biblical times. In the Covenant of Moses, God established a triple sharing of the priestly office, with the High Priest, other Aaronite Priests who assisted in the Temple and the Levitical Priests who assisted the High Priests. In the new covenant, this structure is paralleled by the Bishop who leads a specific diocese and shares his ministry with the Priests, who in turn are assisted by Deacons.
From the beginning, this Sacrament has been reserved to men, chosen from among the people of God to be set aside to be living signs of Christ love for the world. The conferral of this Sacrament is reserved to the Bishop and is passed on through the laying on of hands, as at the first ordination of St. Stephan and his fellow deacons in the Acts of the Apostles.