There are three “holy orders”, each of which can be received only once, and each of which causes a permanent change in a person's relationship to God. The orders are: Deacon, Priest and Bishop. When Jesus appointed his apostles he gave them several commissions: Teach all nations: baptize; forgive sins; heal; cast out demons, etc. He sent them out to preach and teach and heal during his own lifetime. He related their role to the role of shepherds. In the early church, in the Acts of the Apostles, we see how the leaders of the Church saw their role, and how it began to evolve. Deacons were appointed; Peter and the Apostles made rules about how the Church was supposed to be run, and how Christians were to live. Later we have examples of apostles appointing leaders of local communities, the first “bishops”. Elders (presbyters) were ordained to advise and assist the bishop. The roles of the Orders changed with time, and for a long period the order of deacon was conferred mostly on men preparing to be priests. All the orders to some degree participate in governance, leading the assembly at prayer, teaching, preaching, and service.
A Holy Order is conferred upon a man by the laying on of hands by a bishop, who has the role of apostle in a given diocese. A bishop is the only person who has received all three orders.
The grace of Orders configures the person to Jesus Christ; the deacon to Jesus the Servant, the priest to Jesus the Priest, and the bishop to Jesus the Ruler. “Configure” means that Jesus acts through the individual in his particular role. (When someone with Orders says “The Lord be with you” during a liturgical celebration, the response “and with your spirit” is a recognition of this configuration.)
The diaconate is open to married and single men who are in good standing with the Church, and who have been approved for ordination by the bishop. The priesthood is open to men who are not married, who are in good standing with the Church, and who are approved by the bishop. Finally, a bishop is selected from priests, usually from a list of potential candidates generated by diocesan bishops and leaders of religious orders, and finally approved by the Vatican.