Holy Orders is the sacrament through which the mission entrusted by Christ to his apostles continues to be exercised in the Church until the end of time: thus it is the sacrament of apostolic ministry. It includes three degrees: episcopate, presbyterate, and diaconate. CCC 1536

The redemptive sacrifice of Christ is unique, accomplished once for all; yet it is made present in the Eucharistic sacrifice of the Church. the same is true of the one priesthood of Christ; it is made present through the ministerial priesthood without diminishing the uniqueness of Christ's priesthood: "Only Christ is the true priest, the others being only his ministers." CCC 1545

The divinely instituted ecclesiastical ministry is exercised in different degrees by those who even from ancient times have been called bishops, priests, and deacons."Catholic doctrine, expressed in the liturgy, the Magisterium, and the constant practice of the Church, recognizes that there are two degrees of ministerial participation in the priesthood of Christ: the episcopacy and the presbyterate . the diaconate is intended to help and serve them. For this reason the term sacerdos in current usage denotes bishops and priests but not deacons. Yet Catholic doctrine teaches that the degrees of priestly participation (episcopate and presbyterate) and the degree of service (diaconate) are all three conferred by a sacramental act called "ordination," that is, by the sacrament of Holy Orders:

Let everyone revere the deacons as Jesus Christ, the bishop as the image of the Father, and the presbyters as the senate of God and the assembly of the apostles. For without them one cannot speak of the Church.  CCC 1554

"Here I am, send me." (Is 6:8)

From the moment of Jesus' conception in the womb of Mary until his Resurrection, he was filled with the Holy Spirit.  In biblical language, he was anointed by the Holy Spirit and thus established by God the Father as our high priest. As Risen Lord, he remains our high priest. . . . While all the baptized share in Christ's priesthood, the ministerial priesthood shares this through the Sacrament of Holy Orders in a special way. 

Ordination to the priesthood is always a call and a gift from God. Christ reminded his Apostles that they needed to ask the Lord of the harvest to send laborers into the harvest.  Those who seek priesthood respond generously to God's call using the words of the prophet, "Here I am, send me" (Is 6:8).  This call from God can be recognized and understood from the daily signs that disclose his will to those in charge of discerning the vocation of the candidate.

~from the United States Catholic Catechism for Adults