A little information about the Psalms...
Origin and authorship:
The Psalms were likely written and collected before the Babylonian Exile. Interestingly, scholars cannot give a more exact date than the most general. Psalms were not written, as some have proposed, in support of a religious ceremony. The Psalms are songs and poetry. There is no proof that they were or were not intended for liturgical use. We simply know that they were eventually used to that end. Like much song and poetry, it was first an oral tradition and then later codified.
Regarding authorship, “Seventy-three psalms are attributed to David, two to Solomon, Psalm 90 to Moses, 12 psalms to Asaph, 11 to the Sons of Korah, with Psalm 88 to Heman as well, and Psalm 89 to Ethan.” Even these attributions are inexact by contemporary standards. David likely did not write all seventy-three of the psalms attributed to him. Some would have been within the same “school” of psalm writing.
The “types” of Psalms:
Psalms fall under five (or more) types. Each “type” expresses a purpose for its use. There are hymns, laments, thanksgivings, blessings and curses, and wisdom or didactic poems. There are other ways that scholars have gathered psalms together, but in the most basic understanding, this is how one can understand how there is a variety of psalms.
The cultic and liturgical use of the Psalms:
There is no complete example of an ancient Israelite Temple service. There are examples of other cultures' liturgies and some of what may have been intended could be inferred. But most of what scholars understand comes from the Psalms themselves. They could have been intended for specific festivals like Passover. They could have been intended for specific prayer services during the “daily office.” Their use would have been public and private. They would have been accompanied by stringed instruments more often than not.
The arrangement of the Psalms:
The psalms that are in the Biblical psalter are not all the psalms recorded. Some are found in other scripture texts (Ex. 15:1-18 or Isa. 38:10-20). The Old Testament Psalms contain 150 pieces. Psalm 151 is from the Septuagint and the Syriac texts. Other collections merge the psalms together or divide them into separate works. Traditionally, however, the psalter is divided in the Torah into five books. Psalms 1-41, 42-72, 73-89, 90-106, 107-150.
Finally, why “Psalms?”
The word comes from the Greek translation of the Hebrew. They are names after the stringed instrument that could have accompanied them (“psalterion”). But as there are five books of psalms, the original Hebrew is not specific about naming the entire collection. The names for each book reflect their use and nothing else.