Holy Orders is the sacrament which continues Christ's mission through the grace and power given to men to carry out the sacred duties of deacons, priests or bishops.
Bishops (episcopoi) are those who have care of multiple congregations and have the task of appointing, ordaining, and disciplining priests and deacons. They are often called 'evangelists' in the New Testament. Examples of first century bishops include Timothy and Titus (1 Tim. 5:19-22, 2 Tim. 4:5, Titus 1:5).
Priests (presbuteroi) are also known as "presbyters" or "elders." In fact, the English term "priest" is simply a contraction of the Greek word "presbuteros." They have the responsibility of teaching, governing, and performing the sacraments in a given congregation (1 Tim. 5:17, Jas. 5:14-15).
Deacons (diakonoi) are the assistants of the bishops and have the task of teaching and administering certain church functions, such as the distribution of food (Acts 6:1-6).
An extensive account of the teaching of the Catholic Church on the Sacrament of Holy Orders is given inCatechism of the Catholic Church, 1536-1600