Who is Iluvatar from The Silmarillion and what is their importance?

Iluvatar or Eru is the creator of Ea, the Valar, Elves, and Men. After he creates the Ainur, he reveals a theme to them, and they sing in harmony. He shows them his plans for Ea and sends the Valar to Ea to prepare it for his children. His role is that of an omniscient, omnipotent god.