The primary role of the altar server is to assist the priest in the celebration of the liturgy during 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.
The altar server has a privileged place in liturgical celebrations. Those who serve at Mass present themselves to a community. They experience from close at hand that Jesus Christ is present and active in every liturgy. Jesus is present whenever the community gathers to pray and give praise to God.
Therefore in the liturgy they are far more than mere "helpers of the parish priest". Above all, they are servants of Jesus Christ, the eternal High Priest.