In the first age of Middle Earth, the Balrogs (or "Belryg") were Maiar, of the same order as Gandalf, Saruman and Sauron. .
The Balrogs were fiercely loyal to the first dark lord, Morgoth. At least one Balrog, later know as Durin's Bane, survived after the defeat of Morgoth. He fled into the Misty Mountains, where he hibernated until he was awoken by the dwarf miners, lead by Durin. Durin was slain, thus earning the Balrog his name. Durin's Bane remained in hiding within the Mines of Moria, until he was awoken again.
Major Story Arcs
On their journey to destroy the one ring, the Fellowship of the Ring was forced to travel through the Mines of Moria. During a battle with a group of Orcs, the fellowship accidentally reawakened the Balrog. He followed them to the Bridge of Khazad-dûm. Here, the wizard Gandalf faced the Balrog. Gandalf successfully saved the fellowship, but in the process the Balrog and he slew each other, ending the creatures life.
The Balrog has been featured in numerous video game adaptations of J.R.R. Tolkien's The Lord of the Rings trilogy.
The Balrog appeared in Ralph Bakshi's adaptation of Tolkien's first two novels in the Lord of the Rings trilogy. Here is is portrayed as a large, bipedal lion, with the ability to fly via wings.
The Balrog appeared in Peter Jackson's live action adaptations of The Fellowship of the Ring and The Two Towers. He is large, and demon-like in appearance. His look was based of John Howe's drawings of the character.