An altar server is a lay assistant to the Bishop/Priest/Deacon during the Holy Mass or other church service. An altar server attends to supporting tasks at the altar such as retrieving and carrying, ringing the altar bells, etc.
The primary roll of the altar server is to assist the priest in the celebration of the Holy Mass. This is done through specific actions and by setting an example to the congregation by active participation in the liturgy (hymns, responses, etc.), by looking alert and sitting or standing at the appropriate times.
Servers carry the cross, the processional candle(s), hold the book for the priest celebrant when he is not at the altar, carry the incense and censer, present the bread, wine, and water to the priest during the preparation of the gifts or assist him when he receives the gifts from the people, wash the hands of the priest, assist the priest celebrant and deacon as necessary.