Liturgical Ministries
The celebration of Mass is the action of Christ and the People of God -- ministers and congregation. Within the one body of Christ are many gifts and responsibilities. But just as each organ and limb is necessary for the sound functioning of the body (see 1 Cor. 12), so every member of the liturgical assembly gathered by Christ has a part to play in the action of the whole. It is, therefore, of the greatest importance that in all circumstances and on every occasion the celebration be organized so that the priest, deacon, lay ministers, and faithful may all take their own parts. The participation of all is demanded by the nature of the liturgy and, for the faithful, is their right and duty by reason of their Baptism.
In the celebration of the Eucharist, all who are present participate according to their role and function in the Liturgy. A celebration is the work of the whole body of Christ; the ministers and other members of the congregation have a part in the action and have a contribution to make. Each of these special services is performed for the good of the whole and for the glory of God.
PRIEST CELEBRANT:
In the celebration of the Eucharist, Christ is present in the person of the presiding priest. Every celebration of the Eucharist is presided over in the person of Christ by the Bishop or a priest. The Bishop or priest leads the people in praying, in listening and responding to God's word, and in offering the sacrifice through Christ in the Spirit to the Father. He proclaims the message of salvation in preaching the homily and ministers the bread and cup of salvation.
DEACON:
The deacon, whose order has been held in high honor since the early Church, holds, after the priest, the "first place among those who minister in the celebration of the Eucharist." In communion with the Bishop and the priests, the deacon's service for the People of God is the diakonia of liturgy, word, and charity.
LECTOR:
In proclaiming the word of God, lectors exercise their responsibility in the liturgical celebration. God speaks to the faithful through them, and the effectiveness of their proclamation of God's word depends significantly on their conviction, preparation, and delivery.
A psalmist, a cantor, and organist, other instrumentalists, a choir, and a director of music aid the gathered assembly's full participation in singing responses, acclamations, and other texts set to music. These musicians exercise a genuine liturgical function and, by their role, help to add beauty and solemnity to the celebration.
EXTRAORDINARY MINISTERS OF THE EUCHARIST:
Bishops, priests, and deacons are the ordinary ministers of Holy Communion. However, assistance will be given by other Extraordinary Ministers of Holy Communion, either formally commissioned for a given period or, in case of necessity, deputed as needed by the priest celebrant. These ministers serve Christ present in the gathered assembly by ministering his Body and Blood to their brothers and sisters. They also serve the unity of the worshipping community by taking Holy Communion to those members who are prevented from taking part in the congregation.
ACOLYTES/SERVERS:
In addition to the service of instituted Adult Acolytes, assistance at the altar by servers represents a long liturgical tradition. These servers enhance the quality of celebration for the whole gathered assembly by taking part in processions and by ensuring that all items required for the celebration are available at the appropriate moments.
USHERS/GREETERS:
St. Paul instructed the assembled community to "welcome one another as Christ has welcomed you, to the glory of God" (Romans 15:7). It will normally be appropriate for those commonly referred to as "ushers" to exercise their role by welcoming people at the door, providing them with all necessary books and aids, and helping them find their places. The people are coming as invited guests of the Lord himself, to share in his supper as sisters and brothers. They will appreciate this more readily if they are made welcome by representatives of the community and acknowledged informally by their neighbors.
Taken from "Introduction to the Order of Mass: A Pastoral Resource of the Bishops' Committee on the Liturgy"