The Balrogs were spirits of fire seduced by Melkor in the beginning of Arda. In appearance, the Balrogs were man-like, but fire streamed from them, and they were swathed in dark shadows. They carried whips of flame and induced great terror in friends and foes alike. Most of the Balrogs were destroyed in the War of Wrath, but some few escaped over the Blue Mountains and lay hid in Middle-earth. Durin's Bane, the balrog that drove the Dwarves from Moria, was one of these. This Balrog caused the fall of Gandalf but was ultimately destroyed by Gandalf.
(more info and discussion about whether Balrog's had wings or not at: Encyclopedia of Arda)
"Shall we continue to our next creature: The Nazgul?"
