Hell is featured to be a infinite, cavernous realm fit with towers of fire and cliff-sides. Rivers of glowing, flourescent green vomit, freshly harvested from the damned souls and demons that inhabit the realm.
Due to the metaphysical properties of the realm, the landscape and the many demons that inhabit it change to meet the whims of those there, usually to help fit some sort of ironic "punishment" for the soul. Such examples include demons vomiting and deficating into the mouth of those that "spouted filth" and flaming tongues, nipples and genitals for those who did not act upon their desires in life.
The many demons and creatures - including The Great Adversary - are constructs that exist to serve out the purpose the Hell dimension was made for, enacting various tortures onto the souls that exist there.
Unlike in the classic christian interpretation of Hell, the realm is neither eternal nor are any of the souls forced to go and stay there. The majority of souls there came to Hell willingly because they believed in their life that they deserved such torment and were taught that doing so was the natural thing to do, the hundreds of souls moving on to a better afterlife after they asked for Jon's help in escaping.