Psalms is not really a book. It is a collection of 150 songs, prayers, poems and hymns gathered over a very long period of time. There are different types of psalms. Some tell stories. Some are songs of praise. Some are prayers of repentence. Each psalm has its own unique character but they are all deeply emotional and profoundly spiritual. The Psalms are not meant to be simply read, but to be prayed. They encompass the whole range of human emotions from sorrow, lament and depression to joy, praise and celebration. The whole collection of psalms is often referred to as The Psalter.
The compilers of the Psalms divided them into five books: 1-41, 42-72, 73-89, 90-106 and 107-150. The ancient Greek translation of the Bible, the Septuagint (LXX), numbered the Psalms differently than the Hebrew. St. Jerome's Latin translation, the Vulgate, followed the LXX numbering, yet most modern translations use the Hebrew numbering.
The ancient Israelites prayed the Psalms in the tabernacle and then in the Temple from the time of David down to the destruction of the Second Temple in 70 AD. Since then the Jewish people have continued to pray the Psalms. The earliest Christians also prayed the Psalms (see Eph 5:19; Col 3:16) and the Psalms were incorporated into the liturgy of the Mass. The Psalms are still prayed by the Church daily in the Liturgy of the Hours. The 150 Psalms are the basis for the 150 Hail Marys of the Rosary (until the recent addition of the Luminous Mysteries), so that the Rosary has often been called the Little Psalter.
The Levites sung psalms in the Temple. Unfortunately, we do not know the melodies they used nor their methods of praying the Psalms. Yet it is likely that many of the Psalms were sung antiphonally, with one person leading a group in a call-and-response or with two groups of people responding to each other. Early Christian monks adopted these forms of antiphonal prayer of the Psalms. The contemporary liturgy of the Mass also uses an antiphonal psalm. Similarly, the Psalms in the Liturgy of the Hours are often spoken or chanted antiphonally with two "choirs." The Liturgy of the Hours adds a doxology, the "Glory to the Father...," at the end of each psalm.
The Psalms express the delight of the Lord in his people and their delight in him. They show the deep love which exists between the two. The Psalms are both personal and communal. Their varied themes and concerns require us to change our attitudes of prayer to correspond with the particular psalm. St. Augustine taught that "if the psalm prays, pray. If it laments, lament. If it rejoices, rejoice. If it hopes, hope. If it fears, fear. For everything which is written here is a reflection of us." The Psalms mirror human emotions and simultaneously reveal God's heart for us.
Some of the Psalms are prophetic and find their fulfillment in the life of Christ. For example, Ps 22 speaks of his Passion. The New Testament specifically links a few psalms to Christ (e.g. Acts 1:20, 13:33-35). The Psalms' Hebrew poetry does not contain rhyme and meter like English poetry. Rather, the poetry is built on parallelism in which a phrase is paired with a similar or contrasting idea for emphasis. Some psalms have an acrostic structure meaning that the words or phrases are in alphabetical order according to the Hebrew alphabet (e.g. Ps 119).
The Psalms are the key to the spirituality of the Old Testament and they are an essential and permanent part of Christian prayer (CCC 2597).