The Levitical priesthood, the seventy elders, and the priesthood of Melchizedek, all figures of the priesthood of the Old Testament, all prefigured the priesthood of the New Covenant, which Christ established at the Last Supper.  This priesthood is handed down to us today throught Apostolic Succession.
The matter of Holy Orders is the laying on of hands, and the form is the consecratory prayer.  Only a bishop may ordain a deacon, priest or another bishop.
Holy Orders exists in three degrees:  deacon (the diaconate), priest (the presbyterate), and bishop (the episcopate).  Bishops possess the "fullness" of Holy Orders.
The Sacrament of Holy Orders configures the recipient to Christ so that he may serve the Chruch as Christ's instrument.  He is given the mission and faculty (or "sacred power") to act according to his particular order as well as the graces proper to the exercise of it.
(The Sacraments: Source of Our Life in Christ.  The Didache Series).