Starting soon after the releases of the 2 edition rules for Advanced Dungeons &Dragons in 1989, TSR wanted to expand the AD&D universe to include thing besides the traditional Tolkien fantasy settings of their three popular worlds of Greyhawk, Dragonlance, and the Forgotten Realms. These included the SpellJammer outerspace setting, the Gothic horror setting of Ravenloft (an outgrowth of an earlier independent AD&D module), the Arabian Nights setting of Al-Qadim (technically part of the Forgotten Realms world of Toril), and the Dark Sun setting which owes a good deal to Conan the Barbarian.
One of the last of these settings was Planescape. Now Planescape wasn't really a new setting as such. It was more of a translation of the 1 edition AD&D Manual of the Planes to fit with 2 edition rules as well as a rewrite to remove any mention of angels, gods, demons or devils to avoid further criticism about how Dungeons & Dragons was a pathway to Satanism (a part of the overall Satanism panic of the early and mid 1980s).
The Planes and Cosmology of the Universe
Planes are infinite rings but contain borders. You may say that's impossible and well you'd be right but at the same time you'd be completely wrong. It doesn't make sense but at the same time it does.
The majority of 2 edition AD&D setting take place in what is known as the Prime Material Plane (Ravenloft is an exception). These settings are set in solar systems that are encased in crystals. These separate crystals are a way of explaining why magic and other rules operate slightly differently is each campaign setting. Travel between the crystals is difficult relying on certain keys and spelljamer ships with a full herd of giant space hamsters ready to power you along.
There are two other sets of planes, the Inner Planes and the Outer Planes. The Inner Planes sit along what is known as the Ethereal Plane, a plane onto it self. Basically its a shapeless void of mist that one navigates in order to reach other Inner Planes, though getting to the Ethereal plane isn't an easy task in itself requiring a good deal of magic. The Ethereal Plane contains what are known as demi-planes, basically small pieces of matter sitting in the mists. The Ravenloft setting takes place on one of these demi-planes.
The Inner Planes themselves are based on the elements. You have your four basic elemental planes of Air, Earth, Fire and Water set up in a ring with Air and Earth opposite and Fire and Water opposite. Between each of these planes are paraelemental planes which represent the pure essence of their namesakes. Between Air and Fire is Smoke, between Earth and Fire is Magma, between Earth and Water is Ooze, and between Air and Water is Ice. Now take this basic ring and add a plane above and a plane below. The top plane is the Positive Energy plane which represents the energy of life. The bottom plane is the Negative Energy is the plane of death. Between each of the Elemental planes and the energy planes are what are known as Quasielemental Planes which like the paraelemental plances represent the pure essence of their namesakes. Between the Positive Energy plane and Air is Lightening, between the Positive and Fire is Radiance, the Positive and Earth is Mineral, and the Positive and Water is steam. On the Negative side between the Negative and Air is Vacuum, between the Negative and Fire is Ash, between the Negative and Earth is Dust, and between the Negative and Water is Salt.
Where the Inner Planes contain the building blocks of the Prime Material Plane, the Outer Planes contain the building blocks of Philosophy and Free Will. Here the Powers (think gods but not so as not to provoke easily enraged parents) reside tied to Planes that represent the 9 different AD&D alignments as well as some transitional alignment planes. The Outer Planes are attached and can be traveled to by the Astral Plain. Unlike the Ethereal Plane, the Astral Plane is bright and shiny. However while the Ethereal Plane is fairly its mostly safe and boring. The Astral Plain is neither safe nor boring. Portals to the Outer Planes dot the landscape but they are flat and only one side shimmers to alert you that its a portal. But the backside is a portal as well so one can just accidentally trip and find themselves in the middle of the Blood War. In addition a race known as the Githyanki who purposely target travelers of the Astral Plane.
The Outer Planes include the Abyss (Chaotic Evil), the Acheron (Lawful Neutral Evil), Arborea a.k.a. Olympus (Chaotic Good), Arcadia (Lawful Neutral Good), Baator a.k.a. Hell (Lawful Evil), the Beastlands (Neutral Good Chaotic), Bytopia (Neutral Good Lawful), Carceri a.k.a. Tarterus (Neutral Good Evil), Elysium (Neutral Good), Gehenna (Neutral Evil Lawful). The Gray Waste (Neutral Evil), Limbo (Chaotic Neutral), Mechanus (Lawful Neutral), Mount Celestia a.k.a the Seven Heavens (Lawful Good), Pandemonium (Chaotic Evil Neutral) and Ysgard (Chaotic Good Neutral).
The leaves one more plane, one the Primes call the Plane of Concordant Opposition but what those in the know call the Outlands. Its is the Plane aligned with True Neutrality and it is here that Sigil is located. If you recall planes are infinite but have borders. Well those borders exist but it doesn't mean one can actually cross them. The borders of the Outlands are set and a the plane itself is finite across. The Outlands sits in the middles of the Outer Planes and so as you reach the boarder you get closer to a specific plane and the Outlands starts to resemble that plane.
In the middle of the Outlands there is an infinitely tall tower. On top of this infinitely tall tower (yes that's impossible but there you go) sits a ring and inside this ring is the city of Sigil.
Sigil is built on the inside of a ring. Think of a city built on the inside of tire. This means that when one looks up, assuming one can see past the fog a smoke that can occur the city, you just see another part of the city. Technically up and down in context of the Plane is actually right or left but gravity is more important in cases like these than geographical orientation. One thing that can through off new visitors to Sigil is that unlike a planet Sigil's horizon curves in the other direction.
Sigil is as close to the center of the universe as one can be. Not just because it lies in the center of the Outer Planes but because it connects to all points in the universe through portals. Hence one of the other names Sigil is called, the City of Doors. If you know the location of a portal and you have the correct key you can go anyplace in the universe. However neither of those things are particularly easy to find. One problem is that there are three types of portals, ones that move, one are temporary and then one that are permanent. And the keys could be anything and without the key you aren't going anywhere. So sometimes random people accidentally open a portal to Sigil and can't get out. Hence another name for Sigil, The Cage.
Travel between the planes can be dangerous as noted before. It can also be fairly difficult. But travel through Sigil's portals is fairly straight forward once you've got a key and a stable portal to use. And there are some rather valuable things out there on the planes. So there is a lot of powerful things flowing through Sigil. Any Power would love for Sigil to be under their domain. That is where the Lady of Pain comes in. She keeps the Powers at bay and keep Sigil free from overt influence.
The city itself is divided into six wards. The Hive is the slum a Sigil where the poor and unwanted are forced to make their home. The Lower Ward is the industrial center of Sigil with foundries releasing constant smoke to the sky. Given the heat ans smoke its a natural place for portals to the Lower Planes making it even hotter and smokier. The Clerk's Ward is where the city's lower rung bureaucrats and small time artisans live. The Market and Guildhall Wards is where commerce takes place and where the bulk of the city's middle class lives. The final ward is The Lady's Ward where the Lady of Pain herself lives and where the wealthiest citizens of Sigil live.
Alignment is only one facet on one's belief in Sigil. After awhile you see so much that the mind needs a unifying philosophy to explain everything you see and experience. And this is where the factions come in. Residents of Sigil join factions to give their view of the universe guidance and these factions are very important in the functioning of Sigil with certain civil duties falling to different factions. Now these factions have very different views about how the universe is composed from believing that there is one god in all things to the fact that life is a joke and the punchline is we're all dead. These sorts of disputes can easily lead to confrontations but attempts are made to not have it spill out into the streets of Sigil.
Video Game Appearances
In addition to being the setting of Planescape: Torment, Sigil is also referenced in Baldur's Gate II. Players of BGII could partake in a sidequest involving a group of actors from Sigil, and could also purchase special items that belonged to party members from Planescape: Torment.