“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).
Why Is this Sacrament Called “Orders”? The word “order” in Roman antiquity designated an established civil body, especially a governing body. Ordinatio means incorporation into an ordo. In the Church there are established bodies which Tradition, not without a basis in Sacred Scripture, has since ancient times called taxeis (Greek) or ordines. And so the liturgy speaks of the ordo episcoporum, the ordo presbyterorum, the ordo diaconorum. Other groups also receive this name of ordo: catechumens, virgins, spouses, widows . . .
Integration into one of these bodies in the church was accomplished by a rite called ordinatio, a religious and liturgical act which was a consecration, a blessing or a sacrament. Today the word “ordination” is reserved for the sacramental act which integrates a man into the order of bishops, presbyters, or deacons, and goes beyond a simple election, designation, delegation, or institution by the community. Ordination now confers a gift of the Holy Spirit that permits the exercise of a “sacred power” (sacra potestas) which can come only from Christ himself through his Church. The laying on of hands by the bishop, with the consecratory prayer, constitutes the visible sign of this ordination.
Catechism of the Catholic Church The Sacrament of Holy Orders
Becoming a priest or a man or woman religious is not primarily our own decision…Rather it is the response to a call and to a call of love.
Pope Francis, address to Seminarians and Novices, July 6, 2013