What differentiates the Valar from the Maiar?

[Belegorn]

Originally Posted by TheLostPilgrim

What differentiates the Valar from the Maiar? They are both Ainur, correct?

Also, were the Maiar as old as the Valar--did all the Ainur come into being at the same time, in the Beginning? More to the point, would the spirit later known in Arda as Gandalf, have taken part in the Song? Would Gandalf have been there in the Beginning with the rest of the Ainur?

Yes they were both Ainur.

"With the Valar came other spirits whose being also began before the World, of the same order as the Valar but of less degree... their servants and helpers." [Sil. p. 23]

Of the Valar it is said that they are "the Powers of the World" since "their power should thenceforward be contained and bounded in the World, to be within it forever, until it is complete, so that they are its life and it is theirs." [p. 10]

The Maiar were called forth by Manwë and the other Valar, "he called unto himself many spirits both greater and less, and they came down into the fields of Arda and aided Manwë, lest Melkor should hinder the fulfillment of their labour for ever... the Valar drew unto them many companions , some less, some well nigh as great as themselves, and they labored together in the ordering of the Earth and the curbing of its tumults." [pp. 11-12]