The Valar were the fourteen powerful spirits who took physical form and entered Arda after its creation to give order to the world and combat the evils of Melkor. They dwelt originally on the Isle of Almaren, but after its destruction they moved to Aman and founded Valinor.
It is thought that Melkor was at one time a Vala, but he was forced out of their number.
Manwë and Melkor fought, and Manwë called other spirits to help him in his battle. Among these were the other Valar and the Maiar. Melkor withdrew from the battle, and the others continued their creation of the World. But Melkor saw this, and returned to fight for control of Arda.
The Valar have no fixed shape, but often take the shapes of Men and Elves, or they can remain invisible.