Balrog: Also called Valaraukar, the Balrogs were originally Maiar corrupted by Melkor (Morgoth). Both Balrog and Valaraukar mean 'Demon of Power'.
They were man-shaped, but fire and smoke streamed from them, and they were shrouded in shadow. They normally carried flaming whips. The question of whether they have wings is still hotly debated.
When Morgoth was attacked by the Valar in the War of Wrath, some of his Balrogs fled into Middle-earth and made their home in the deep places of the world. One such place was the Mines of Moria.
When Ungoliant trapped Melkor in one of her webs, it was the Balrogs that came to save him.
However, the Balrogs were not indestructible. Gothmog the Balrog, their Lord at one time, was killed by Ecthelion of the Fountain. Another was slain by Glorfindel of Gondolin. Durin's Bane, the Balrog of Moria, was killed by Gandalf the Grey.