Hell is a region characterized by the intense pain and suffering of its inhabitants. Accounts vary on its consistency, size, temperature, and location, but the one universal constant of the concept of Hell is that it contains some level of supernatural force, usually characterized as "the Devil".
Occupants of Hell
In Judeo-Christian theology and works which derive their basis from it, Hell contains the Devil and legions of his underlings, called demons. The souls of the damned are relegated to Hell as punishment for their transgressions.
Most popular works derive their inspiration for hell from the works of Dante Aligheri, most notably his Inferno, in which he describes the various "layers" of Hell and the specific sins of the occupants thereof.
Location in the Omniverse
There are an infinite number of locations in the Omniverse which fit the description of Hell by the nature of the Omniverse itself. Some are considered extra-dimensional or otherwise untethered to the place which sends them souls. This is correct for the most part, but some instances of "Hell on Earth" have been recorded, where the conditions of Hell for a particular planet somehow break through a dimensional barrier to affect the source of its energy.