Since the beginning, the ordained ministry has been conferred and exercised in three degrees: that of bishops, that of presbyters, and that of deacons. The ministries conferred by ordination are irreplaceable for the organic structure of the Church: without the bishop, presbyters, and deacons, one cannot speak of the Church (cf St. Ignatius of Antioch, Ad Trall. 3,1).
Source: Catechism of the Catholic Church (Vatican.va)