The Psalms, the prayer of the assembly
The Psalter is the book in which the Word of God becomes man's prayer. In other books of the Old Testament, "the words proclaim [God's] works and bring to light the mystery they contain." The words of the Psalmist, sung for God, both express and acclaim the Lord's saving works; the same Spirit inspires both God's work and man's response. Christ will unite the two. In him, the psalms continue to teach us how to pray.
Source: Catechism of the Catholic Church (Vatican.va)