From WikiPilipinas: The Hip 'n Free Philippine Encyclopedia
Lay Minister refers to Roman Catholics who have an official or semi-official function in the Church but who have not received Holy Orders nor professed religious vows.
The Second Vatican Council taught that the laity's specific character is secularity, i.e. as Christians who live the life of Christ in the world. The role of Christian lay people is to sanctify the created world by directing it to become more Christian in its structures and systems : "It belongs to the laity to seek the kingdom of God by engaging in the affairs of the world and directing them according to God's will" (Lumen Gentium). The laity are full members of the Church, who fully share in Church's purpose of sanctification, of "inner union of men with God," (CCC 775) acting with freedom and personal responsibility and not as mere agents of the hierarchy. Due to their baptism, they are members of God's family, the Church, and they grow in intimate union with God, "in" and "by means" of the world. It is not a matter of departing from the world as the monks and the nuns do that they sanctify themselves; it is precisely through the material world sanctified by the coming of the God made flesh, i.e. made material, that they reach God. Doctors, mothers of a family, farmers, bank tellers, drivers, by doing their jobs in the world with a Christian spirit are already extending the Kingdom of God. According to the repeated statements of Popes and lay Catholic leaders, the laity should say "we are the Church," in the same way that the saints said that "Christ lives in me."
The involvement of the laity has taken diverse forms including participation in the life of the parish, unions of prayer, confraternities, communes, guilds, lay apostolates, Catholic Action, secular institutes, and lay ecclesial movements.
The role of the laity in the Church includes lay ministers. Also, as a result of the clergy shortage, members of the laity have had to perform some of the roles previously performed by priests.
 Instituted Lay Ministries
Certain ministries were established by the Christian communities since early times for the proper ordering of worship and for the service of the People of God as the need arose. These ministries were entrusted to and conferred upon Christian lay persons by a special rite invoking God's blessing and designating these persons in a special rank or class for the performance of some function in the Christian community.
Some of these functions were intimately linked with the celebration of the liturgy and later these became preparatory stages in the reception of holdy orders -- what came to be termed minor orders. In the Latin-rite Church, these minor orders were the offices of porter [or doorkeeper], lector [or reader], exorcist, and acolyte. They were termed minor orders in comparison with the pre-Vatican II major orders of the subdiaconate, the diaconate, and the priesthood.
After the Second Vatican Council, the offices of the lector or reader and of the acolyte, which are especially connected with the ministries of the Word and of the altar, were retained and adapted to contemporary needs. These offices were henceforth to be referred to as ministries rather than as minor orders, and were no longer to be regarded as reserved to candidates for holy orders. Nonetheless, in keeping with the venerable tradition of the Catholic Church, current ecclesiastical legislation limits the institution of lectors and acolytes to baptized men only. Moreover, the conferring of these permanent and established ministries is not accompanied by the right of remuneration from the Christian community.
 Instituted Reader or Lector
The proper office of the instituted reader or lector is to read the Word of God in the liturgical assembly. Consequently, it is the lector's task : (1) to read the texts from Sacred Scripture [but not the Gospel, which is reserved to deacons and priests] at Mass and at other sacred functions; (2) in the absence of a psalmist, to recite the psalm between the readings; (3) in the absence of a deacon or cantor, to announce the intentions at the prayers of the faithful; (4) to direct the singing and the participation of the worshipping congregation; (5) to instruct believers in the worthy reception of the sacraments; (6) when necessary, to prepare the persons who are temporarily appointed to read the Scriptures in liturgical celebrations.
Mindful of his office, the lector is expected to acquire an increasing sweet and living love and knowledge of Sacred Scripture, so as to make him a more perfect disciple of the Lord.
In parish communities, the functions of an instituted reader or lector are often exercised by readers and commentators, who are often commissioned but not according to the officially promulgated rites of institution or installation to ministry conferred by the ordinary.
 Instituted Acolyte
The proper office of the acolyte [or, in some places, subdeacon] is to assist the deacon and to minister to the priest. Consequently, it is the acolyte's task : (1) to attend to the service of the altar by assisting the deacon and the priest in liturgical celebrations, especially the Eucharist; (2) as an extraordinary minister, to distribute holy communion in the inability of the ordinary ministers [i.e. deacons, presbyters, bishops] of communion, due to illness, old age, or other pastoral function and in the event of a great number of communicants which would unduly prolong the Eucharistic celebration; (3) in the absence of ordinary ministers of the Eucharist, to expose and to repose the Blessed Sacrament for the veneration of the faithful, but without benediction; (4) to instruct lay persons temporarily appointed to assist the clergy at the altar as [missal]book-bearers, cross-bearers, candle-bearers, or other similar liturgical functions.
Mindful of his office, the instituted acolyte is expected to fervently participate in the Eucharist and to nourish and deepen his understanding of it. He is also expected to familiarize himself with all that pertains to divine worship, endeavoring to understand its spirit and inner meaning, thus becoming an example of gravity and reverence to all. He is to have a sincere love for the mystical Body of Christ, the People of God, especially for the weak and the sick.
In parish communities, the functions of an instituted acolyte are often exercised by commissioned extraordinary ministers of the Eucharist, which are popularly but inaccurately termed "lay ministers". These extraordinary Eucharistic ministers are referred to variously in different dioceses, e.g. assistant ministers of the Eucharist, extraordinary ministers for the distribution of communion, special ministers of the Eucharist. Some of the functions of acolytes are also exercised by altar servers.
 Original Source
Pope Paul VI, Apostolic Letter Ministeria Quaedam on First Tonsure, Minor Orders, and the Subdiaconate, 15 August 1972.