Eru is the greatest being, greater than the Valar. He is known as Ilúvatar to the Elves. He created the Ainur and the Maiar from his thought. He placed the errand of creating music that reflected the feelings of himself on them. They sang and played in unison and managed, until Melkor, greatest of the Ainur, sought to make the music flow to his own will. Three times he was interrupted, and three times the music was retained by Ilúvatar, until he ended it altogether. He then spoke to them, and to Melkor, saying that no music shall be played against him, or they shall be proven to be feeble in the might, skill, and majesty of Ilúvatar. This made Melkor ashamed, and he developed a secret anger within himself. Ilúvatar then instructed the the Ainur to construct the world and the Maiar to aid them, in accordance with his song, which served as a prevision. The Ainur learned from Ilúvatar's song and vision much of what would unfold, but some things they cannot see, and all is known but to Ilúvatar himself.
Ilúvatar allows the Ainur and the Maiar to embark on their duty in the world envisioned and to enforce what he desires to be. He makes no interference in the history of Middle-earth, except in the Fall of Númenor.