From an article by Bishop Jovan (Puric), Church Chant – God’s Gift to the Angels, that I’m too lazy to translate in its entirety, an excerpt:
THE THEOLOGICAL ELEMENTS OF CHANTING
Chanting is made up of three elements: word, melody and rhythm.
It is only man who has been gifted with word, hence he can offer his voice and thereby imitate his creator. St. John Chrysostom says that our word proceeds from our soul just as the Logos proceeds from the Father (Letter to Euthropio, PG 52, 403).
The word also has a social dimension. It is form of communication of one man to another. (See St. John Chrysostom’s Commentary on the Epistle to the Ephesians, PG 62, 126).
Through words man communicates with God as well. However, we cannot worthily express the divine mystery, hence we resort to chanting (See St. John Chrysostom’s Homily on the Ascension of our Lord, PG 52, 775). St. Nil of Sinai thinks the same: “God must be glorified with words, respected with deeds and worshiped with thoughts” (Sayings, PG 79, 1249c).
In Orthodox chant word and melody are inseparable. The melody enhances the word, the “clothing” of the word. The word expressed through melody says much more than the naked word. But the melody is not an artificial complement but, like the word, proceeds from the human nature, that is, it’s a gift.
God, in creating the world, created the specific rhythm of moving in that world, like the rhythm of time (for example, the shift from day to night, the movement of the planets, etc.). And so rhythm appears as a part of divine harmony, then rhythm reveals the presence of divine beauty in the world. It has it ethical dimensions as well: respecting the divinely established order and participating in it, man conforms with the divine rhythm, that is, he conforms towards God.
Rhythm is inseparable from word and melody.
SONGS AND REVELATION
In the Old Testament there are three different kinds of poetry: song, psalm and hymn.
The song is mentioned for the first time in the book of Exodus, when Moses sings a song of thanksgiving to God for taking them across the Red Sea. This song, besides that of thanksgiving also has a doxological character.
St. Basil the Great calls songs “high theology” (Commentary on Psalm 49, PG 29, 305C), since God’s actions are revealed through song (God’s revelation to the world and man’s response to that revelation).
The term psalm is first mentioned in the First Book of Kings (16,18). When Saul was besieged David made him psalms and thereby freed him from the “evil spirit” (here we see a therapeutic dimension of chanting).
Regarding the content, psalms are very similar to songs.
Hymns are the third part of chanting and are found very frequently in the Old Testament. The problem with hymns is that they are very difficult to differentiate from both songs and psalms, and so at time they are called thus.
Hymns are of a descriptive and thanksgiving characteristic, and their content is, for the most part, dogmatic. St. Athansius refers to psalms as hymns. (Homily on the suffering and crucifixion of our Lord, PG 28, 193A).