The Maiar (also called the People of the Valar; sing. Maia) were those of the lesser Ainur who chose to enter Eä. They tended Arda under the direction of the Valar, though some – notably Sauron and the Valaraukar (Balrogs) – were seduced away from their allegiance by Melkor.
Though the Maiar were far greater in number than the Valar, only eight of those most loyal are named:
Ilmarë – the Handmaid of Varda
Eönwë – the Herald of Manwë
Ossë, Uinen, and Salmar – these Maiair served Ulmo, Lord of the Sea
Melian – in the following of Vana and Estë, she came to Middle-earth, wedded Thingol, and passed on a strain of the Ainur to the Children of Ilúvatar via their daughter Lúthien
Arien – originally served Vana but was chosen to steer the Sun through the Heavens
Tilion – follower of Oromë who afterwards became the Steersman of the Moon
Additionally, the Five Wizards (Istari) were Maiar of Valinor in their origins.