The Hebrew title of the Psalms also means “hymns.” Psalm 72:20 refers to all preceding Psalms as, the prayers of David. Both items are surprising and yet understandable. To be sure, at first glance, the Psalms contain exclusively neither hymns nor prayers. In spite of that the didactic poems or the songs of lament are also basically hymns, for they serve to glorify God, and even those Psalms which do not contain any address to God (e.g., Psalms 1, 2, 78) may be called prayers, for they serve to submerge in God’s will and purpose the one who prays them. The word Psalter originally referred to a musical instrument, and was first used only in a secondary sense as a designation for a collection of prayers which were offered to God in the form of songs.
The Psalms, as we have them today, are for the most part set to music for liturgical use. Vocal parts and instruments of all sorts work together. Again it is David to whom the particular liturgical music is ascribed. As at one time his harp playing drove away the evil spirit, so the sacred liturgical music is such an effective power that occasionally the same word can be used for it as is used for prophetic preaching (1 Chronicles 25:2). Many of the titles of the Psalms which are difficult to understand are directions for the musicians. This also applies to the “Sela” which often occurs in the middle of a Psalm and which apparently signals an interlude. The Sela indicates that one must be still and quickly think through the words of the Psalm; for they demand a quiet and restful soul, which can grasp and hold to that which the Holy Spirit there presents and offers. (Martin Luther)
The Psalms were probably most often sung antiphonally. They were particularly well suited for that through the verse form, according to which the two parts of a verse are so connected that they express in different words essentially the same thought. This is called parallelism. This form is not simply accidental. It encourages us not to allow the prayer to be cut off prematurely, and it invites us to pray together with one another. That which seems to be unnecessary repetition to us, who are inclined to pray too hurriedly, is actually proper immersion and concentration in prayer. It is at the same time the sign that many, indeed all believers, pray with different words yet with one and the same word. Therefore the verse form in particular summons us to pray the Psalms together.