All Christians share in Christ's priesthood.
All Catholics, indeed all Christians, share in the priesthood of Christ through their baptism into his death and resurrection.
In the New Covenant we have two kinds of priests. Those who have received the sacrament of Holy Orders and those who belong to the priesthood of the faithful (also called laity).
Priests offer sacrifice. That is their job: to be a mediator between God and man by offering sacrifice. This is very clear in the Old Covenant where the sacrifice offered is the blood of animals. In the New Covenant, Christ, the Lamb of God “entered once for all into the Holy Place, taking not the blood of goats and calves but his own blood” (Hebrews 9:12). The revelation of the Cross of Christ is that true love is sacrificial love. In this way Christ has offered an acceptable sacrifice to God the Father making it possible for the sacrifices of the faithful to participate in his one saving sacrifice.
Sacrifice is the center of Christian life because joined to the sacrifice of Christ...
the lives of the faithful, their praise, sufferings, prayer, and work are united with those of Christ and with his total offering, and so acquire a new value. (Catechism of the Catholic Church, 1368)
In the Church there are two different kinds of priesthood which “differ from one another in essence and not only in degree” (Vatican II, Dogmatic Constitution on the Church, 10). They are not two levels of priesthood, but are really different kinds of priesthood.
Jesus gave a profound gift to the eleven disciples who, except for John, had all betrayed Jesus at his passion.
breathed on then and said to them, ‘Receive the Holy Spirit. Whose sins you forgive are forgiven them, and whose sins you retain are retained.’ (John 20:22-23)
At the Last Supper Jesus had given these men the office of the priesthood also called Holy Orders. “The ministerial priest, by the sacred power he enjoys, teaches and rules the priestly people; acting in the person of Christ, he makes present the Eucharistic sacrifice, and offers it to God in the name of all the people” (Vatican II, Dogmatic Constitution on the Church, 10). Priests offer their entire lives in service and speak the words of absolution to forgive sins in the sacrament of confession.
Holy Orders describes the sacrament which has three degrees: bishop, priest, and deacon. The fullness of Holy Orders rests in the office of bishop. Consecration as a bishop makes him a real and legitimate successor of the apostles. Priests are ordained to serve the entire Church and like the bishop exercise this ministry through the forgiveness of sins. Deacons are ordained either transitionally (as part of the path to priestly ordination) or permanently to serve at the altar and exercise the ministry of the word (preaching and teaching). They also take on specific efforts of charity, service to the sick, and pastoral care. Deacons are not ordained to forgive sins (except in the administration of Baptism) and so they cannot confect the Eucharist, hear confessions, or anoint the sick.
The Mass then is the source of life for the Catholic Christian. It is our hope for heaven and the source of our strength and union with God here on earth. As Pope Benedict XVI recently wrote: “The Eucharist, since it embraces the concrete, everyday existence of the believer, makes possible, day by day, the progressive transfiguration of all those called by grace to reflect the image of the Son of God” (Sacramentum Caritatis, 71).
Bishops, Priests and Deacons help the laity in their vocation to holiness
With such a “powerful means of salvation, all the faithful, whatever their condition or state, are called by the Lord, each in his own way, to that perfect holiness whereby the Father Himself is perfect” (Vatican II, Dogmatic Constitution on the Church, 11). On our own our sacrifices have no power to save, but Christ made them powerful. The laity especially are given the great gift of the Holy Eucharist through the words and at the hands of the priest at Mass. Our participation in Mass is important because “it is necessary that the faithful come to it with proper dispositions… that they cooperate with heavenly grace lest they receive it in vain” (Vatican II, Constitution on the Sacred Liturgy, 11).
Everyone wants to go to heaven… but nobody wants to die to get there
Each member of the faithful has a role to play in offering this “acceptable sacrifice” to God the Father. In wealthy countries many of the faithful have a tendency to be lazy about the faith. Since Vatican II the distinctly Catholic spirituality of sacrifice has largely disappeared among the faithful. This is not what the Council intended. If we seek to live forever in heaven we must become people who live like God does, loving unselfishly both God and neighbor. There is no room for selfishness in the authentic Christian life. This is the “why” behind such Catholic practices as abstinence from meat, fasting, almsgiving and even prayer. The spiritual power behind these practices is not rooted in a hatred of the person or the goods of the world. It is rooted in love. Christian life isn’t about what can I get from God. It is about what I can give as an “acceptable offering.” Christ died for our sins and we must respond to this gift by dying to our self as Jesus taught that “whoever loses his life for my sake, will save it.” (Luke 9:24) This is the logic of Holy Orders through which a rich offering is given in service to God and His people. This same logic applies to all the faithful as well who are called according to their state in life to offer their life in loving service in their homes, workplace, school; wherever God places them in the world.
The English word "priest" comes from the Greek word presbuteros, which means "elder."
PRESBUTEROS => PRiESBUTEROS
This is the etymology behind the English term "priest," much like the etymology behind the English term "weaver" is the German word "weber," which means: "a textile worker who makes cloth out of raw material." It isn't coincidental that weaver happens to look a lot like weber, just as it isn't coincidental that priest looks a lot like presbyteros.
Throughout the New Testament, we see presbyters as those elders who are ordained through the laying on of the hands.