The Sacrament of Holy Orders is the rite by which a man is ordained as a deacon, priest or bishop. The bishop administers this sacrament by the laying on of hands, which confers on a man the grace and spiritual power to celebrate the Church’s sacraments.
The priesthood has its roots in the Old Testament, beginning with a priestly paternity that passed from father to first-born son and later with the Levites as the priestly tribe. Jesus, of course, is the culmination - the High Priest.
While all of the baptized share in Christ's priesthood, ordained priests, bishops, and deacons share in that priesthood in a special way. When a priest celebrates the sacraments [e.g., consecrates the host (Eucharist) or forgives sins in the sacrament of Reconciliation], it is not the man who is doing these things - it is Christ workingthrough the man.
Ordained ministers are the continuing link in the apostolic tradition of the Church. Just as the pope is the successor to Peter, the bishops are successors to the apostles and are supported by priests and deacons. (See these verses for some instances of the early church establishing leadership and conferring ordination: Acts 14:23, 20:17; Phil 1:1; Tit 1:5-9.)
In every age consecrted men and women must continue to be images of Christ the Lord, fostering through prayer a profound communion of mind with him.--Pope John Paul II Apostolic Eshortation Vita Consecrata