Limbo is both another dimension and a nexxus - it stands outside the multiverse, so to speak, but can be used to access any time or place, if one can control the stepping disks that are generated there.
In and of itself, Limbo is best described as a "demon dimension". The landscape is harsh and forbidding, given to extremes in temperature and environment. Limbo's former ruler, a sorceror named Belasco, shaped it to resemble what he remembered of Hell from Dante's Inferno, and its current ruler, Magik, has yet to make any long-lasting changes. It's permanent inhabitants include "demons"; inhuman entities of various types, some sentient and some not, and various mutated versions of Earth animals. Long term visitors to Limbo generally take on demonic attributes as well, even if these are not thrust upon them.
Different areas of Limbo feature different ecosystems; some of the prominent ones include a volcanic realm, an arid one, and one that could be best said to resemble Siberia in winter. There is also an acid lake, which its best to not drink from.
The Ruler of Limbo, currently Magik, inhabits a castle that looks as if it could have been taken straight from an old horror movie. Its interior is always changing, but inevitably features a massive throne room. A metal skeleton is pinned to one of the walls by a lance; if the demons know whose body is was, they're not telling.