Netherrealm usually appears to be like a realm that is composed of thousands of different layers. Hell is usually "the darkest region of reality", and it is often inhabited by demons and monsters. The Netherrealm is usually comparable to Hell in the Abrahamic faiths. It seems to look like an eternal volcanic "continent", that contains many volcanoes and lava pits. See more details at http://mortalkombat.wikia.com/wiki/Netherrealm.