Maiar (Quenya, singular Maia) are entities that entered Arda to assist the Valar with shaping the world. Many Maiar were not named, though they were supposedly numerous. The Maiar had two leaders, known as chiefs. The first chief was Eönwë, banner-bearer and herald of Manwë. The second was Ilmarë, the handmaid of Varda.
Each Maia was associated with one or more Valar. The Maiar were less powerful than the Valar, but they did carry some resemblance. The most powerful of the Maiar were capable of opposing a Vala. Some of the Maiar sided with Melkor before much was known about his evil, one of whom was Sauron.
In about T.A. 1100, the Valar sent several Maiar to Middle-earth to help fight the evil of Sauron. Known initially as the Istari and later simply as wizards, these beings had skills of hand and mind but were cloaked in the form of men, seemingly old and of great vigor. Their mission was to guide elves and men by gaining trust and spreading knowledge, rather than by ruling them with fear and force.