In Arda - the world in which the events of "The Lord of the Rings" take place - there is a whole pantheon of deities - the Valar. These are the most powerful spirits who organized the world after its creation and were appointed by Eru Ilúvatar as its rulers.
The most powerful of the Valar - Melkor - went against the will of the creator, tried to organize the world at will and fought with the rest of the Valar, elves and humans.