What’s the difference between a psalm and a canticle?
Our term psalm comes from a Greek word literally meaning the twanging of a harp or plucking of a stringed instrument. Canticle derives from the Latin word for a little song. As both definitions suggest, we’re talking about sung material, particularly sacred songs. The main difference between the two is not style, but placement. Psalms are found entirely within the Book of Psalms. Canticles are songs located anywhere else in Scripture.
The psalm collection, known as psalmody or the Psalter, contains musical directions that indicate at least a third of the 150 poems within the book were intended for stringed, flute, or harp accompaniment. Some were apparently set to music everyone knew: read notations like “the hind of the dawn” the way our hymns might recommend “Finlandia” or “Pange Lingua”. The word selah appears 71 times in the collection. We don’t know what it means, but the choir certainly would have. Internally, some psalms also carry subtitles that distinguish them as songs, hymns, or prayers. This doesn’t imply the others are not songs or prayers. It’s just that these entered the collection with these titles, the way “The Lord’s Prayer” is obviously not the only prayer of Jesus included in the gospels. In the Jewish Bible, the entire collection we call psalms is known by the Hebrew word for hymns. The bottom line is there’s no indication any of these poems were intended merely for recitation, as we often do.
King David, traditionally considered the author of the whole book of psalms, is internally attributed with at least 73 of them. (Other manuscripts ascribe 84 to David). The others bear the names of other composers. Biblical evidence suggests David was a poet, composer, and musician, not to mention the organizer of the liturgical cult of the Temple. If he didn’t actually compose half of the Psalmody, he was its primary original sponsor.
Canticles have a broad authorship. Song of Songs, AKA Canticle of Canticles, was traditionally ascribed to King Solomon. The subject matter is a series of love songs, which suited Solomon’s reputation as a renowned lover. However, most scholars see multiple and later author involvement. Important Old Testament canticles include those attributed to Miriam, Moses, Deborah, Hannah, and Judith. New Testament canticles include the Benedictus of Zechariah, the Nunc Dimittis of Simeon, and of course the Magnificat of Mary. More recent canticles include those of Francis of Assisi and John of the Cross.
Source: https://vocationnetwork.org/en/blog/questions_catholics_ask/2018/12/whats_the_difference_between_a_psalm_and_a_canticle