The book of Psalms is a collection of 150 hymns and prayers. Many of these were composed by King David.
The Psalms cover a wide variety of topics:
Sometimes the mood is joyful, even jubilant. Other psalms are filled with despair, loneliness, and worry. It's often been said that no matter what your mood, you'll find a psalm to match it.
The Psalms are both national and personal. That is, some of the hymns are about national issues—like war, poverty, or the coming Messiah; while others express the intimate feelings of just one person.
The Psalms were quoted by Jesus and other writers of the New Testament. So the book of Psalms has been treasured by the Christian church from its very beginning.