The Sacrament of Holy Orders is the sacrament in which the ministry of Christ as Head of the Church is exercised, and it is the sacrament through which apostolic succession is handed on in the Church. Through Holy Orders, men are conformed to Christ in such a way that Christ becomes really present in them and working through them when they perform the liturgy and the sacraments.
Read and Study: United States Catholic Catechism for Adults Chapter 20 (pgs. 261-275)