A deacon, at his ordination, receives the authority to proclaim the Gospel during mass, and the duty to teach the faith, and the duty to assist the bishop and the bishop’s priests in the sacrifice of Divine Worship services (namely, the Mass, the Divine Liturgy, the Qurbana, or Qorbono). Specifically, in all rites, he leads the prayers of the faithful (the Roman PotF, the Byzantine and Coptic litanies) and proclaims the gospel. He also makes solemn promises of celibacy and obedience. (Remembering: Celibacy means not being able to contract a new marriage - if already married, he remains so, and if not, he will never marry.)
He normally receives, in the Roman Church, the faculties to preach, to lead services in the absence of a priest*, to baptise, to be the church’s witness to marriages, and to bless sacramentals. He may be tasked to teach any level of religious education by the bishop, but especially frequently is assigned baptismal preparation, marriage preparation, and RCIA. Many RC deacons also serve in both hospital and prison ministries.
In the eastern churches, the faculties are fewer, usually including only preaching and distributing communion to the faithful (as EMHC’s are allowed in only a couple of the Eastern Churches in Union with Rome). Again, the educational role is widespread. It’s more common for deacons to function as parish administration staff in the eastern churches, and provide more assistance outside of the sacramental role. The deacon’s role in the Divine Liturgy is more visible than in the west, and the duty no less profound.
Slavic Byzantine Deacons, especially Ukrainian and Ruthenian, may be blessed to lead certain services in the absence of a priest, sometimes including “Typica with Communion”… no layman may lead that service, and when done by a priest, it’s called “The Divine Liturgy of the Presanctified Gifts.” (Presanctified is done on weekdays of Lent and St. Phillip’s Fast - aka advent - instead of a consecratory Divine Liturgy. Typica with communion is used in missions or when the priest is sick. )
- Almost all services except the Mass may be lead by a deacon, *if his bishop lets him. * A communion service lead by a deacon traditionally counts as a Divine Worship Service, both East and West. In the Slavic Byzantine Tradition, it’s called Deacon’s Typica with Communion.