"The Sacrament of Apostolic Ministry by which the mission entrusted by Christ to his Apostles continues to be exercised in the Church through the laying on of hands. This sacrament has three distinct degrees or 'orders': deacon, priest, and bishop. All three confer a permanent, sacramental character" (Catechism of the Catholic Church,536).
"The whole Church is a priestly people. Through Baptism all the faithful share in the priesthood of Christ. This participation is called the 'common priesthood of the faithful.' Based on this common priesthood and ordered to its service, there exists another participation in the mission of Christ: the ministry conferred by the sacrament of Holy Orders, where the task is to serve in the name and in the person of Christ the Head in the midst of the community" (Catechism, 1591).
There are two sacraments at the service of communion: Holy Orders and Holy Matrimony. Both of these sacraments confer a special grace directed not towards the salvation of the one who receives the sacrament, but to the salvation of those who are served by the one ordained or married. In Baptism and Confirmation, we are consecrated or set apart from the world by God and for God; in Holy Orders and Holy Matrimony we receive another consecration. Bishops, priests, and deacons are consecrated to feed the Church by the Word and grace of God, and spouses are consecrated for the duties and dignity of marital love and family life.
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, and "ordination" 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.
Bishops and presbyters share by different degrees in the one ministerial priesthood of the New Covenant; by their consecration, bishops and priests are configured to the Lord Jesus in such a way that they can act in His Person in the sacred liturgy and stand in the Person of Christ, Head and Bridegroom of the Church. The ministerial priesthood has the task of representing Christ the Head of the Church before the whole assembly and also of acting in the name of the whole Church when offering to God the prayer of the Church. Deacons are ordained unto a ministry of service, but not to the priesthood. Deacons assist bishops and priests in the celebration of the sacred mysteries, in works of charity, in blessing marriages, in the proclamation of the Gospel, in administering baptism, and in presiding over funerals. Read sections 1533-1600 of the Catechism of the Catholic Church for a fuller explanation of the Sacrament of Holy Orders.