Limbo is sometimes referred to as "Otherplace" and it is a pocket dimension where the laws of nature are mainly magic. Its appearance is that of a solid land where gravity still has a hold but the sky suffers from an absence of stars. The general landscape changes with each ruler usually very mountainous terrain.
In Dante's Divine Comedy Limbo is described as the first circle of hell.
Limbo in Marvel comics:
Limbo is first shown in Uncanny X-men #160, when its ruler Belasco kidnaps Illyana Rasputin to Limbo and when the X-men rescue her, she is seemingly aged several years. In reality, during the moments they lost her grasp, those several years really passed for her, such is the magic of time in Limbo.
Those that dwell within its boundaries are a vast variety of demons of all shapes and sizes who all have some sort of magical skill. Time is peculiar in this dimension as the present is non-existent and can only be stated to be so by an individual. Limbo is also spotted by magical "stepping discs" which travel through time as well as space allowing a traveler, if he or she could control them, to be anywhere, anywhen instantly. As an adolescent, Illyana developed the mutant ability to control these discs, giving her a natural affinity to Limbo. The farther the jump in time or space she tried to make with the discs though, the less accurate she was able to be.
The beings of Limbo will blindly serve their master who is most commonly a Supreme Sorcerer, but if not kept in check they may betray their overlord or plague other realities. The appearance of Limbo can change quite drastically based on whatever its ruler fancies or even sometimes based on their unconscious desires.
Also, if a human enters Limbo it is considered inevitable that eventually they shall gain demonic physical aspects (i.e. horns, tails, hooves, etc) and though this transformation will give them mastery over sorcery it typically costs the person their souls (examples are Magik and the Bogeyman
Limbo is kept alive by what appears to be a giant animal heart hidden deep beneath its surface in a cavern. But truthfully it is simply a small deposit of promethium that would cause Limbo to disappear if it was removed.
Magik's Soulsword has become a symbol of the ruler of Limbo.
Rulers of Limbo:
Belasco: He is not as savage as the other demons and has a more human appearance, perhaps meaning that he was once human, he is the longest-running known ruler of Limbo and recently tried to claim it as his own again and kidnapped the students at the Xavier Institute but this attempt was halted.
Witchfire: The daughter of Limbo's former ruler Belasco. She recently tried to claim her fathers place at the Hell Lords' table by taking over Limbo for herself. She was defeated by the newly resurrected Magik and the New X-men team. She is currently trapped in the Elder God's dimension along with several Bloodstones.
Illyana Rasputin: She was kidnapped by Belasco as a child and after learning from him she eventually overthrew him to become the ruler of Limbo. She has the ability to leave at will but chose to rule it instead, until she died of the Legacy Virus. She has recently returned when Belasco tried to reclaim Limbo and has now retaken the throne.
Sym and N'astirh: These two were of the greatest demons inhabiting Limbo, S'ym being the master of physical power and N'astirh of magical power. The two became servants of Belasco and then Illyana but betrayed Illyana after her temporary absence. They then crossed into Earth where they wreaked havoc after fooling Illyana into making a permanent portal into the Earthly realm (see Inferno). N'astirh died int his attempt and S'ym was later killed by Darkoth. They were recently seen as the servants of Illyana yet again now that she has retaken the throne, but this has been left unexplained.
Doctor Doom: Victor von Doom temporarily took over Limbo when he wished to steal its source, the deposit of promethium, which he wished to use to permanently power his Earthly nation of Latveria.
Darkoth: Darkoth ruled Limbo for an unknown period of time and at this time also possessed the Soulsword that he had taken from Doctor Doom.
Margali Szardos: She stole the Soulsword from Kitty Pryde so that she could become the greatest follower of The Winding Way and also used it to take over Limbo.
Amanda Sefton: Jimaine Szardos took the Soulsword from her mother and became the ruler of Limbo for some time, taking the name Magik as Illyana had before her. She eventually lost control of Limbo when Belasco banished her, after Illyana regained control she found she was unable to return to Limbo.