"As each has received a gift, use it to serve one another, as good stewards of God's varied grace." 1 Peter 4:10
Eucharistic Minister, or more properly "Lay Eucharistic Minister LEM", is used to denote a lay person who assists the priest in administering the sacraments of holy communion, the consecrated bread and wine. They may also take the sacraments to those who are ill, or otherwise unable to attend Mass.
Liturgical Assistants have an important ministry during the Mass that facilitates the flow of the liturgy and helps lead the congregation to a deeper awareness of the presence of God. They assist the clergy with various liturgical tasks and play a significant role in leadership of the altar servers as the master of ceremonies.
An altar server is a lay assistant to a member of the clergy during a Christian liturgy. An altar server attends to supporting tasks at the altar such as fetching and carrying, ringing the altar bell, among other things.
Sacristan is a person appointed for the care of the sacristy, sacred vessels, vestments, and other articles required by the ceremonial for any liturgical function
Lector or reader can mean someone who in a particular liturgy is assigned to read a Biblical text other than the Gospel. (Reading the Gospel at Mass is reserved specifically to the deacon or, in his absence, to the priest.)
An usher is a person who helps ensure a smoothly running church service and who ministers to people in a variety of practical ways. The specific responsibilities of an usher vary, but their duties usually include greeting people as they arrive for the service, assisting people with special needs, and collecting the offering.