What is Sunday Liturgy?
The Divine Liturgy is the apex of Orthodox Christian worship, and takes the same form worldwide, although in different languages and sometimes somewhat different musical traditions. It includes Holy Communion for Orthodox Christians in good standing who have prepared themselves through Confession and fasting with the blessing of the Priest. As described by Fr. Thomas Hopko in The Orthodox Faith:
"The Divine Liturgy is the common action of Orthodox Christians officially gathered to constitute the Orthodox Church. It is the action of the Church assembled by God in order to be together in one community to worship, to pray, to sing, to hear God’s Word, to be instructed in God’s commandments, to offer itself with thanksgiving in Christ to God the Father, and to have the living experience of God’s eternal kingdom through communion with the same Christ Who is present in his people by the Holy Spirit. The Divine Liturgy celebrated by the Orthodox Church is called the Liturgy of Saint John Chrysostom.... Although it is generally the practice in the Orthodox Church today to allow non-Orthodox Christians, and even non-Christians, to witness the Liturgy of the Faithful, it is still the practice to reserve actual participation in the sacrament of Holy Communion only to members of the Orthodox Church who are fully committed to the life and teachings of the Orthodox Faith as preserved, proclaimed and practiced by the Church throughout its history."
Liturgies often last approximately 1 hour and 15 minutes, not including the Reading of the Hours which starts 20 minutes before the Liturgy.
What is a Vespers Service?
"The service of Vespers takes us through creation, sin, and salvation in Christ. It leads us to the meditation of God’s word and the glorification of his love for men. It instructs us and allows us to praise God for the particular events or persons whose memory is celebrated and made present to us in the Church. It prepares us for the sleep of the night and the dawn of the new day to come. On the evening before the Divine Liturgy, it begins our movement into the most perfect communion with God in the sacramental mysteries."
Vespers Services are shorter services that are meant to prepare Orthodox faithful for the following day's Divine Liturgy. Reader's Vespers also precede Reader's Services by a day and are are usually shorter than standard Vespers.
What is a Reader's Service?
As written by Fr. John Whiteford, GoodGuysWearBlack.org:
"In case a priest is not available, with his blessing or that of his bishop, deacons, subdeacons, readers, and even laity can lead a modified service for the faithful. We call such services 'Reader’s Services.' Reader’s Services are useful in established parishes because they allow parishes to maintain a regular worship schedule even when their priest is on vacation, ill, or otherwise unavailable. The availability of Reader’s Services also opens up the possibility for dedicated laity, deacons, and lower clergy to start and maintain missions and for families that are away from any Orthodox parish to maintain an Orthodox liturgical life."
Reader's Services are generally shorter services, conducted by a member of the lesser clergy or laity.