The word psalm comes from the Greek word, psalmoi, a song sung to a harp, originally derived from psallein, to play on a stringed instrument. The Book of Psalms consists of 150 of these lyrical poems.
The main characteristic of this Hebrew poetic form is parallelism, a balanced thought usually presented in two parts. The first part states a thesis and the second part reflects the thesis, usually in one of three ways: repeating the idea, adding to the idea, or opposing the idea.
(Example Psalm 121:6 "The sun shall not smite thee by day, nor the moon by night.")