Holy Orders is the sacrament by which bishops, priests, and deacons receive the grace and authority to perform their sacred duties through a sacred rite called ordination.
Instituted by Christ himself, Holy Orders is administered through the laying on of hands by the Bishop while saying a specific consecratory prayer asking God for the outpouring of the Holy Spirit and His gifts. The ordained bishop, priest or deacon is given the ability and duty to serve the Church through his preaching, teaching and celebration of the Sacraments.
There is only one Sacrament of Holy Orders, but three levels: Bishop, Priest, and Deacon.
Bishop: The first level is that of bishop, which is what Christ himself bestowed upon His Apostles: the episcopate. A bishop is a man who is ordained to the episcopate by another bishop and stands in a direct, unbroken line from the Apostles, a quality known as “apostolic succession.” Ordination as a bishop confers the grace to sanctify and govern others, thus all episcopal ordinations must be approved by the Pope.
Priest: The second level of the Sacrament of Holy Orders is the presbyterate, which is what we most commonly call the priesthood. Priests serve as co-workers of the bishop and act in the person of Christ to offer Mass (Eucharist), preach the Gospel, and celebrate the Sacraments of Baptism, Reconciliation (Confession), Anointing of the Sick, and Matrimony (marriage).
Deacon: The diaconate is the third level of Holy Orders. Deacons assist the bishops and priests, but they also minister on behalf of the Church themselves. In addition to serving at the altar, deacons proclaim the gospel during liturgies, and they can minister the Sacrament of Baptism and lead the wedding ceremony for the Sacrament of Matrimony. However, the root of the name for deacons means service (diaconia), so they are called to be of service to others in a wide variety of ways. In fact, all bishops and priests are first ordained as transitional deacons, which reminds them all levels of Holy Orders are called to the humility of servant-leadership after the model of Jesus Christ. Married men can become permanent deacons, but once a man has received the Sacrament of Holy Orders he cannot marry (or remarry if his wife dies).