ModernAngel has it slightly wrong --A careful reading of the Ainulindalë will show this.

The Ainur are the beings that Ilúvatar created at the very beginning, and sang in the Music of the Ainur.

The Valar are those of the Ainur who fell in love with the Vision of Ilúvatar and entered its realization, Arda (Earth). Hence the term Valar, which means "Powers of the Earth".

So -- The Valar are all of the Ainur who entered Arda, including the Maiar, and including Melkor. Melkor's evil minions were Maiar that he swayed to his servitude before any of them entered Arda.

All that said, it appears to be common usage to refer to the fourteen most powerful of the Valar, which MA listed, as "the Valar". However, some of the Maiar are revered as highly as the Valar -- esp. Ossë and Uinen.