The Sacrament of Holy Orders is the continuation of Christ's priesthood, which He bestowed upon His Apostles; thus, the Catechism of the Catholic Church refers to the Sacrament of Holy Orders as "the sacrament of apostolic ministry."

In the Sacrament of Holy Orders, or Ordination, the priest being ordained vows to lead other Catholics by bringing them the sacraments (especially the Eucharist), by proclaiming the Gospel, and by providing other means to holiness.

 In the phrase which means to incorporate someone into an order. In context, therefore, a  holy order is simply a group with a hierarchical structure that is set apart for ministry in the Church.  In the Sacrament of Holy Orders, a man is incorporated into the priesthood of Christ, at one of three levels: the episcopate, the priesthood, or the diaconate.

For Catholics, the church views typically that in the last year of seminary training a man will be ordained to the "transitional diaconate."  This distinguishes men bound for priesthood from those who have entered the "permanent diaconate" and do not intend to seek ordination as a priest. Deacons, whether transitional or permanent, receive faculties to preach, to perform baptisms, and to witness marriages. They may assist at the Eucharist or the Mass, but are not the ministers of the Eucharist. After six months or more as a transitional deacon, a man will be ordained to the priesthood. Priests are able to preach, perform baptisms, witness marriages, hear confessions and give absolutions, anoint the sick, and celebrate the Eucharist or the Mass. Some priests are later chosen to be bishops; bishops may ordain priests, deacons, and bishops.