Helheim is the realm of the dead and is named after its ruler, Hel. This is the realm where the souls of those that did not die valiantly in battle are sent to. So succumbing to old age or illness automatically meant that you would be sent off to this ghastly place. Naturally, oath-breakers and criminals would also go here.
Hel, the daughter of Loki, is the queen of Helheim. She is very powerful (even more powerful than Odin) and is the only one that gets to decide if someone is returned to the realms of the living or not. Garm is her guardian hellhound and makes sure that no one leaves the place without her approval.
Amid the cursed souls is a dragon, called Nidhogg. He gnaws at the roots of the Yggdrasil tree and feasts on the bodies of oath-breakers.