The supreme task which Christ had to fulfill was his priestly work of atonement which he completed as mediator between God and man. By the union in himself of humanity and divinity Christ is by nature the mediator. As a man from among men, Christ is our mediator with the Father; yet he is also capable of offering a worthy sacrifice to God because, by virtue of the union of his human nature with the Second Person of the Godhead, his human actions have in infinite value. In this fullest sense, the priesthood belongs to Christ alone.
Source: © Copyright 2009 Catholic Online
THE CHURCH THUS TEACHES:
Order is a true sacrament instituted by Christ who ordained the Apostles at the Last Supper.
It is administered by the laying on of hands and the key phrases of the ordination preface. Only a Bishop can validly ordain. Order is a purely ecclesiastical concern. The effect of the sacrament of order is to impart the Holy Spirit and to impress an indelible character, which permanently distinguishes those in orders from the laity.
The laity also has a part in Christ's priesthood, but in another manner. The office of Bishop is above the priesthood (which in turn is above the diaconate) and gives special powers of consecration.
To the priesthood belong the celebration of Holy Mass and the power of forgiving sins. The subdiaconate belongs to the priesthood and diaconate to the 'major orders.' In addition, the four 'minor orders' were instituted by the Church. Conditions for the valid reception of order are baptism and being of the male sex.