In Hellgate: London, the idea of 'Hell' is intentionally distanced from that most commonly known in Judeo-Christian religions. Instead, Hell is the name given to the alternate dimension from where the invading demonic horde has poured into our world.
Do not be mistaken though- Hell is still the fiery, brimstone-ravaged and tortured Hell that we know from popular culture, festooned with Demon-spawn and governed by devlish overlords with nought on their minds but the destruction of everything in their path.
It is unclear at this stage whether there is a polar opposite to Hell that assists our characters (ie. a "Heaven"), but it is unlikely, on the basis that only the Templar Faction resembles anything akin to the traditional "Holy" warriors.
A walk through Hell[]
It is known to date, from screenshots and trailers released by Flagship Studios, that characters will be able to extend their adventure into "pockets" of Hell, and will be able to explore the barely-liveable plains of Hell for short periods of time.