SakranMM told you most everything. Just to amplify:
In Orthodoxy, the diaconate is one of the three orders of the sacramental priesthood, and those ordained as deacons are considered to be full members of the clergy, and, thus, they only serve with the blessing and under obedience to a Bishop.
Furthermore, in parish life, deacons serve with the blessing and under the direction of the parish pastor, who is a priest (never a deacon). Deacons, as their name suggests, are called to serve, i.e. assist in the liturgical and catechetical life of the parish. However, they never truly lead it. Thus, pretty much every Orthodox service -- even something as "minor" as Vespers -- cannot be fully and completely served without the priest. Certain parts would have to be omitted.
So, within Orthodoxy, the deacon would only do a baptism in extremis, could not celebrate the Sacrament of Marriage, etc. However, the deacon has a major role to play in ALL Orthodox services and sacraments. As SakranMM indicated, that usually entails intoning the petitions, censing, reading the Gospel, participating in processions, etc.