Also Hell, Halja, Hella
"One which hides"
"One who covers up"
In Norse Mythology, Hel was the goddess who resided in the lowest of the nine worlds, Niflheim. There, she reigned over Helheim ("House of Hel") from her hall Eljudnir (the home of the dead). With her manservant Ganglati and her maidservant Ganglot (both translate as "tardy"), she presided over the dark, cold and misty place where those who did not die in battle or who were wicked came after dying (people who died heroically, of course, went to Valhalla). Though it seems to have begun as a place of forgetfulness, in later tales, once in Helheim, the dead were sentenced by Hel to remain in one of nine levels of the afterlife, some of which being very much like heaven and the lowest being dreary, cheerless places filled with constant hunger and pain.
Hel was the daughter of the trickster god Loki and the giantess Angrboda and sister to Fenrir and Jormungand. Through no fault of her own, she was born so hideous and grotesque that the other gods shunned her. She is typically described as having been born with her skeleton on the outside of her body, both black and white or with the face and trunk of a hag and the mottled legs of a decomposing corpse. In one account, Odin, disturbed at her appearance and fearful of the children of Loki, banishes her to the underworld, but in another, Hel herself, saddened by the horror she inspires in the other deities, asks Odin for a realm of her own and is given Niflheim.
There, not even gods could escape the power of death, as is evidenced by Hel's refusal to give Balder, who was killed accidentally, back to Odin and life. From her throne, which represented the death bed, the goddess watched over her lands which were surrounded by the impassable river Gjoll that flowed from the spring Hvergelmir. Other safeguards protected Helheim and ensured that there could be no escape: the entrance was guarded by Garm, a monstrous hound, and the maiden Modgud, who stood on the golden path which led to the underworld. The giant Hraesvelg ("corpse eater") also sat at the edge of the world and watched over Helheim, making the wind blow.
Sometimes, the goddess would mount her three-legged horse and ride across Midgard, bringing with her pestilence, disease, famine and illness, all leading to death. But Hel was not always harsh, and would occasionally allow the dead to come back to life under conditions. It was she who told Hermod that if everyone is each of the nine worlds grieved for Balder, he would be let go (a plan sabotaged by Loki).
At Ragnarok, she will bring an army of the dead woken by the third cock's cry to join her father, Loki, and the giants on the final battlefield. Garn breaks free of his bonds and confronts Tyr. Helheim, like Asgard, disappears and the souls of the dead are moved to various heavens such as Andlang and Vidblain or halls like Gimle's hall, Sindri (which will house the souls of the virtuous) and Nastrand (which will house the souls of murderers and oath-breakers). Unlike her siblings, we do not know how Hel meets her fate on the field of Vigrid.
Sources:
www.pantheon.org
http://www.suite101.com/article.cfm/1417/88621