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.
(On the institution and mission of the apostolic ministry by Christ, see above, no. Ff. Here only the sacramental means by which this ministry is handed on will be treated.)
Source: Catechism of the Catholic Church (Vatican.va)