From the Ultimate Encyclopedia of Mythology, which was a very thoughtful gift from my brother:
Fenrir, or Fenris, according to German mythology, was the son of the mischief-making god Loki and the frost giantess Angrboda. He was the devouring wolf, the beast of Ragnarok, the doom of the gods. His was "an axe-age, a sword-age, a wind-age, a wolf-age, before the wrecking of the world." Odin, the chief of the gods, was destined to become his victim.
Kidnapped by the gods and brought to Asgard where they could keep an eye on him, Fenrir was so savage that only the war god Tyr dared to feed him. At first, Odin was uncertain about the wolf, but when the Norns, the goddesses of destiny, warned him about his own fate, he decided that Fenrir should be restrained. No chain, however, was strong enough to hold the animal. Finally, the dwarves made a magic fetter called Gleipnir from strange materials such as the roots of a mountain and bird's spittle. Although it seemed to be a silken ribbon, Fenrir would not have it round his neck unless one of the gods put his hand between his jaws as pledge that it was as harmless as it seemed.
Tyr was the only one prepared to risk his hand, and all the gods laughed when the wolf bit it off on finding that the chain could not be broken. Fenrir was then secured to a rock and his mouth was kept open by a sword so that he could not bite.
When freed from captivity at Ragnarok, Fenrir was a fearsome spectacle. His vast mouth gaped so wide that the lower jaw touched the ground and the upper one reached the sky, and Odin was swallowed by him.