Holy Orders is the sacrament in which a baptised man receives the authority and ability to share in the particular mission that Christ entrusted to his apostles. There are three orders of this sacrament: episcopate (bishops), presbyterate (priests) and diaconate (deacons).
Holy Orders come from Jesus Christ, who chose twelve men whom he called ‘apostles’. He gave them the commission and power to govern, teach and sanctify (Mt 18:18; Mt 28:19-20; Lk 22:19).
The apostles conferred Holy Orders on their successors, the bishops. These in turn ordained further bishops, priests and deacons through the laying on of hands and the prayer of consecration according to the rite of the Church.
Jesus taught that the call to Holy Orders is his initiative rather than ours, “You did not choose me, but I chose you” (John 15:16). The call is discerned and freely accepted by the man who receives it. The Church tests this call and prepares the candidate for Holy Orders by means of spiritual, human, academic and pastoral formation, usually in a seminary.
This article is originally from ‘CREDO: The Catholic Faith explained’ by CTS.
Follow this link from Catholic about.com for a summary and overview! http://catholicism.about.com/od/beliefsteachings/p/Sac_Holy_Orders.htm