Lesser spirits of the
Ainur, as related by
Bilbo Baggins (via his interpreter,
J.R.R. Tolkien) in his
Translations from the Elvish. About a dozen Maiar are named in the
Valaquenta and a few more in
Quenta Silmarillion. An early version of
The Silmarillion,
The Book of Lost Tales, names a few more. Unnamed Maiar are legion, and at times, Middle-Earth seems crammed full of enough spirits to satisfy any
animistic society.
Created by Ilúvatar at the very beginning of things, they participated in the Ainulindalë, then followed the fourteen greatest Valar into Iluvatar's realization of the music, Eä, or Arda, of which Middle-Earth is a part. In Arda, they occasionally clothed themselves in forms resembling the Children of Ilùvatar.
The Maiar had varying abilities and intellect. Olórin's wisdom and compassion are counterbalanced by the erratic Tilion, and both are a contrast to Sauron. Some dwelt in Valinor with the Valar, others lived in Middle Earth, especially servants of Yavanna watching over the kelvar and olvar. Some, such as the Istari, were sent from Valinor on missions to help the people of Middle Earth.
Each of the Maiar attatched his or her service to one or another of the Valar. Sadly, many were swayed by Melkor, adding to his Discord and marring the Music.