Ilúvatar (Q. ‘All-father’), also known as Eru (the One), was the all-powerful God of Arda.
With the Flame Imperishable, he created the Ainur and revealed to each of them a part of his mind. Revealing to them the three great themes of the Ainulindalë, the Ainur began to sing for him. (For more on the Creation, see: Ainulindalë.)
He then made Eä – the World and All That Is. The Ainur were given the option to go into Eä and fashion it to their liking; these Ainur, known as the Valar, controlled the shaping of Arda. They independently worked to fulfill the Music, but some things were known only to Ilúvatar – such as the creation of Men and Elves (the Children of Ilúvatar), the destiny of Men, and the End. Ilúvatar himself rarely intervened: he sanctified the creation of the Dwarves by Aulë, and later changed Arda at the Valar’s request after the Númenoreans landed on Aman. In The Letters of J.R.R. Tolkien, the author himself has stated that when Gandalf fell in battle with the Balrog, it was beyond the power of the Valar to resurrect him.