The Maiar (singular: Maia) are among a class of beings from J. R. R. Tolkien's high fantasy legendarium. Supernatural and angelic, they are "lesser Ainur" who entered "Eä" in the beginning of time. The name Maiar is in the Quenya tongue (one of several constructed languages) that comes from the Elvish root maya- "excellent, admirable". Maiar sometimes refers to all the Ainur who entered Eä, the "Creation", but more often to the lesser powers among them: "Maia is the name of the Kin of the Valar, but especially of those of lesser power than the 9 great rulers" wrote Tolkien.
In the Valaquenta, Tolkien wrote that the Maiar are "spirits whose being also began before the world, of the same order as the Valar but of less degree". In the Valaquenta, it is also written that many Maiar associated themselves with a Vala; for example, Ossë and Uinen, who ruled the Seas, acted under Ulmo, while Curumo, who came to be known in Middle-earth as Saruman, was with Aulë. Sauron also was with Aulë before being corrupted by Melkor.
Being of divine origin and possessing great power, the Maiar can wander the world unseen or shape themselves in fashion of Elves or other creatures; these "veils", called fanar in Quenya, could be destroyed, but their true-being could not. Rarely did the Maiar adopt their visible forms to elves and man, and for that reason, very few of the Maiar have names in their tongues, and the elves do not know how many of the Maiar exist.
Melkor (known in Sindarin as Morgoth), the evil Vala, corrupted many Maiar into his service. These included Sauron, the main antagonist of The Lord of the Rings, and the Balrogs, his demons of flame and shadow. These are called in Quenya Úmaiar.
Melian was a Maia who went to Middle-earth before the First Age, where she fell in love with the Elven-king Elu Thingol, King Greymantle, and with him ruled the kingdom of Doriath. When war with Morgoth came to Doriath, she used her powers to guard and defend it with a protection called the Girdle of Melian (List Melian in Sindarin). She had a child with Thingol, a daughter named Lúthien, said to be the fairest and most beautiful of all the Children of Ilúvatar.
In about T.A. 1100, the Valar sent several Maiar to Middle-earth to help contest the evil of Sauron. They had great skills of hand and mind but were cloaked in the guise of men, seemingly old but of great vigour. Their mission was to guide elves and men by gaining trust and spreading knowledge, not by ruling them with fear and force. They were known as the Istari, or Wizards, and included Gandalf the Grey (Olórin or Mithrandir; later Gandalf the White), Saruman the White (Curumo or Curunír; he later called himself Saruman of Many Colours), Radagast the Brown (Aiwendil), and two Blue Wizards (Alatar and Pallando) who are mentioned in passing but do not appear in Tolkien's narratives.