The Psalter, also known as the Book of Psalms, is a book of the Hebrew Bible and the Christian Old Testament. It is the longest book in the Bible, containing 150 psalms or songs of praise, prayer, and meditation. The psalms are attributed to King David and other authors, and are believed to have been written over a period of several centuries. They are divided into five books, each with its own theme and purpose. The psalms are used in both Jewish and Christian liturgies, and are often sung or chanted during religious services.