A psalm is a religious song in the Bible. While you can read psalms as prayers, they are meant to be sung.
The Book of Psalms is a section of the Christian Old Testament and the Hebrew Bible that includes 150 individual psalms. The word psalm, which is pronounced with a silent p, comes from the Greek word psalmos, "song sung to a harp," and its root, psallein, "play a stringed instrument." While they're not often backed by a harp these days, psalms are frequently sung with musical accompaniment in churches and temples.