By the sacrament of Holy Orders particular members of the church community are ordained to share in a special way in the preiesthood of Christ. Their role is to represent, to teach and to govern the christian community.
A bishop is a successor of the apostles with the responsibility for preaching the word of God, celebrating the sacraments and governing the church of a particular area.
A priest is appointed to co-operate with the bishop in the work of teaching and celebrating the sacraments.
A deacon is a appointed to co-operate with the bishop and priests in teaching, in the liturgy and in organising works of charity.
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: bishop, priest, and deacon.
'Catechism of the Catholic Church'