The Maiar were Ainur, like the Valar, but of a lesser degree. The singular form of the word is Maia.

The Maiar were the servants of the Valar but before the creation of the world, they too participated in the Ainulindalë or the Music of the Ainur.

The Maiar were numerous, their exact number never being told, but they were named rarely in the histories of Men for they were concerned little with the mortal lands and mortal folk. They were most concerned with the Valar in Aman.

Chief among the Maiar were Ilmarë the handmaid of Varda, and Ëonwë the herald of Manwë. But, best known among the Children of Ilúvatar were Ossë, Uinen.

In later years there were other Maiar who were sent to Middle-earth to aid the Children of Ilúvatar against one of their own, Sauron. They were called the Istari (Wizards) and they were not known to be of the Ainur by many in Middle-earth. Those of the Istari who are best known are Saruman, Gandalf, and Radagast.
Encyclopedia entry originally written by atalante_star