Antiphons consists of one or more psalm verses (or sentences from Holy Scripture), alternating with verses which contain the fundamental thought of the psalm.
The first three hymns of the Divine Liturgy are called the antiphons. On regular Sundays, the first two antiphons are taken from the Old Testament Book of Psalms, Psalm 102/103 (Bless the Lord, O my soul) and Psalm 145/146 (Praise the Lord, O my soul). The Third Antiphon, comes from the New Testament, the Beatitudes.
Following the second antiphon, a hymn by the Emperor Justinian, Only-begotten Son, is always sung. It is a hymn of faith in the divinity of Christ and his incarnation, crucifixion, and resurrection as "one of the Holy Trinity" for the salvation of men.
They are called the antiphons because they were sung by the two choirs, each responding antiphonally to the other. (The alternating verses of the other Choir are now omitted.)
Following the 1838 reform, the Greek tradition (except the Athonite monks who kept the old order) replaced the Psalms and Beatitudes with brief appeals to the Theotokos or to Christ. The Russian tradition continue to sing, each Sunday, the two noted psalms and the Beatitudes. They are replaced by other antiphons only at great feasts or on weekdays.
Some differences between Greek and Russian divine services and their significance Basil Krivoshein, Archbishop of Brussels and Belgium The Orthodox Faith Fr. Thomas Hopko, Dean Emeritus of St. Vladimir's Seminary, Crestwood, NY.