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 (CCC 1536). It includes three degrees: episcopate (bishops), presbyterate (priests), and diaconate (deacons). It 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 (CCC 1538).
Since the sacrament of Holy Orders is the sacrament of the apostolic ministry, it is for the bishops as the successors of the apostles to hand on the "gift of the Spirit," the "apostolic line." Validly ordained bishops, i.e., those who are in the line of apostolic succession, validly confer the three degrees of the sacrament of Holy Orders.(CCC 1576). The laying on of hands by the bishop, with the consecratory prayer, constitutes the visible sign of this ordination.
The Church confers the sacrament of Holy Orders only on baptized men (viri), whose suitability for the exercise of the ministry has been duly recognized. Church authority alone has the responsibility and right to call someone to receive the sacrament of Holy Orders.
Effects of the sacrament
This sacrament configures the recipient to Christ by a special grace of the Holy Spirit, so that he may serve as Christ's instrument for his Church. By ordination one is enabled to act as a representative of Christ, Head of the Church, in his triple office of priest, prophet, and king.
The sacrament of Holy Orders, like Baptism and Confirmation, confers an indelible spiritual character and cannot be repeated or conferred temporarily (CCC 1581 - 1582).
For more information
Sacraments of Healing (CCC Part 2, Section 2, Chapter 3, Article 6)