An altar server is a lay assistant to a member of the clergy during a religious service. Altar servers attend to supporting tasks at the altar such as fetching and carrying, ringing a bell etc. An altar server usually has no formal training and holds no formal religious position.
In the Roman Catholic Church, altar servers are usually young people who help the priest and deacon during the liturgy, espeically the Mass. They have several supporting duties to carry out, such as carrying the processional cross and candles, carry the incense and thurible, holding the missal for the priest celebrant when he is not at the altar, assisting the priest when he receives the gifts from the people, washing the hands of the priest before the prayer over the gifts, and assisting the priest celebrant and the deacon as necessary. Altar servers wear the alb or the surplice during a liturgy, although they can also wear street clothes.
Altar boys are young male altar servers. Formerly, only young men, whom the Church wanted to recruit for the priesthood, and seminarians, who needed the training, were altar servers, and so altar boy was the term for all servers. One reason why the phrase altar boy has ceased to be standard is that recently girls have been allowed to serve in this capacity. At the same time an increasing number of adults are serving at the altar, especially at solemn services in cathedrals or basilicas. In the Roman Catholic Church, altar servers that are studying to become priests or deacons might actually be acolytes, which is a necessary preparatory office before ordination. As part of their training, an acolyte might supervise children and other adults who are altar servers.
In Episcopal churches, all who serve in the above positions are called acolytes.