Short answers, pulled from the story.
Hel is an afterlife realm in Norse mythology and paganism, ruled by a being of the same name. Both the dead and, in certain accounts, living figures travel there. The realm is located in the ninth of the Nine Worlds, within Niflheim.
Hel is situated in the ninth of the Nine Worlds, in a region called Niflheim. One of the three roots of the world tree Yggdrasil leads down to it, according to the poem Grímnismál. To reach it, a traveler must go 'down and to the North,' crossing the river Gjöll on a bridge with a golden roof.
The being who rules Hel also bears the name Hel. Odin cast her into the realm and made her 'ruler over Nine Worlds.' She presides over a great hall called Éljúðnir and can grant lodging to those who died of disease or old age.
The word Hel descends from the Proto-Germanic noun haljō, meaning 'concealed place' or 'the underworld.' That form comes from the Proto-Indo-European root kel- or *kol-, meaning 'to cover, conceal, save.' It shares this ancestry with Modern English 'hell' and, through a related form, with both 'hall' and 'Valhalla.'
After riding nine nights through dark valleys on Sleipnir, Hermóðr crossed the river Gjöll on a golden-roofed bridge guarded by Móðguð. He leapt the Gates of Hel and found Baldr sitting in a seat of honor inside the hall. Hel gave Hermóðr an offer for Baldr's release, but the attempt ultimately failed, with Loki bearing the blame.
Valhalla receives warriors who died in battle. Hel receives those who died of disease or old age, according to Snorri Sturluson's Prose Edda. The scholar Hilda Ellis Davidson suggested this distinction may have been Snorri's own invention rather than ancient tradition.