ru Ilúvatar is not actually one of the Valar. He is the Supreme Being, the Holy, the One. He is the higher power to whom the Valar turn to when the troubles of the world are greater than they can handle. Elves and Men call on him when they need great aid or when making an oath that cannot or will not be broken.
In the beginning of the world, in a time before Time, only Eru existed until he created the Ainur, who sprang forth from his thoughts. He imparted to them his vision of the World, Middle-earth, and they understood his thoughts . They grew and became more powerful through song. Through the Ainur and their Great Music, he created the world, Eä, the World that Is.
The Ainur who became the Valar created by Ilúvatar are numbered fourteen. The Lords are seven and they are Manwë, Ulmo, Aulë, Oromë, Mandos, Lórien, and Tulkas. The Queens are seven also and they are Varda, Yavanna, Nienna, Estë, Vairë, Vána, and Nessa. Melkor is no longer counted among the Valar because of his evil ways and the evil he did to the World of Ilúvatar.
Eru is best likened to the Judeo-Christian G-d, in that he is the Supreme Being of the World, above all other powers. The major difference is that instead of creating all the world by himself and of himself, he used the Ainur, the beings of his thoughts, to realize the vision he had for the world.