Holy Orders is the sacrament through which the mission entrusted by Christ to his apostles is exercised in the Church until the end of time. It is the sacrament of apostolic ministry. It includes three degrees: episcopate, presbyterate and diaconate (Catechism of the Catholic Church, par. 1356).
The word ordo or order is taken from the Roman times, when it designated a group of people. It was mostly used to refer to a governing body. The Latin word ordinatio means incorporation into such an ordo.
The Catholic tradition has numerous such orders: the order of catechumen; the order of virgins; the order of spouses; the order of widows; religious orders that denote groups of monks or nuns who live according the same religious rule. There also is the ordo episcoporum (bishops), the ordo presbyterorum (priests) and the ordo diaconorum (deacons).
The Latin word for ordination, or ordering, was used to mean the incorporation of a person into a specific order. These ordinations could be elaborate or simple ceremonies comprised of a consecration or blessing.
Today the word ordination is only used when referring to the sacramental act by which a man is integrated into the order of bishops, presbyters, or deacons. Through the laying on of hands, a consecratory, and an anointing for bishops and presbyters, the ordaining bishop confer a gift of the Holy Spirit that allows for the celebration of the sacraments proper to the respective order of bishop, priest and deacon.
Bishops, priests and deacons are ordained for sacred ministries that differ, one from the other.
A bishop, by virtue of his ordination, is the direct successor of the Apostles. His principle task is to teach, sanctify and shepherd the members of the Church.
A preist is ordained to assist the bishops in their ministry, and are tasked with celebrating the Eucharist, administering the Sacraments and spreading the Good News.
A deacon is ordained to assist the bishops and priests. They assist at the Eucharist, distribute Holy Communion, baptize, bless marriages, proclaim and preach the Gospel, preside over funerals and above all, dedicate themselves to ministries of charity.