The Catechism of the Catholic Church states that the ordained ministry or ministerial priesthood is at the service of the baptismal priesthood. The ordained priesthood guarantees that it really is Christ who acts in the sacraments through the Holy Spirit for the Church. The saving mission entrusted by the Father to His incarnate Son was committed to the apostles and through them to their successors: they receive the Spirit of Jesus to act in His name and in His person. The ordained minister is the sacramental bond that ties the liturgical action to what the apostles said and did and, through them, to the words and actions of Christ, the source and foundation of the sacraments."
"The whole Church is a priestly people. Through Baptism all the faithful share in the priesthood of Christ. This participation is called the 'common priesthood of the faithful.' Based on this common priesthood and ordered to its service, there exists another participation in the mission of Christ: the ministry conferred by the sacrament of Holy Orders, where the task is to serve in the name and in the person of Christ the Head in the midst of the community."
"The ministerial priesthood differs in essence from the common priesthood of the faithful because it confers a sacred power for the service of the faithful. The ordained ministers exercise their service for the People of God by teaching, divine worship, and pastoral governance."
Since the beginning, the ordained ministry has been conferred, which by ordination is irreplaceable for the organic structure of the Church and is exercised in three degrees: 1. bishops 2. presbyters and 3. deacons.
The Catechism of the Catholic Church, para. 1120, 1591-1593
For more information about these ministries, click on these links to the vocation websites for priests and deacons.