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.
Catechism of the Catholic Church 1536
In the sacrament of Holy Orders, a man is ordained a priest, deacon, or bishop.
A priest promises to live a life of celibacy and of obedience to his local bishop. He is given the command to teach, minister, and govern: to teach the people of God and bring them closer to God, to minister the sacraments, and to govern the people, typically through parish ministry.
A deacon may be “permanent” or “transitional”. A permanent deacon is a man who may or may not be married and who has committed his life in service to the church. A transitional deacon is one who intends to become a priest. This is typically the stage one year prior ordination to the priesthood. A deacon, whether permanent or transitional, may baptize and may officiate at weddings.
A bishop is a priest ordained to the special ministry of Shepherd of the people. A bishop typically oversees a Diocese.