Planescape: Torment is based in the AD&D (Advanced Dungeons and Dragons) world of Planescape. The best-known AD&D world is arguably the Tolkien-esque Forgotten Realms, a medieval-type universe where Dragons run wild, Wizards wield powerful magic, and Elves are in constant strife with the Orcs. The Forgotten Realms has developed extensively over the years and is now more or less defined. If you were to ever join an AD&D campaign, chances are good that it will take place in the Forgotten Realms. Planescape, on the other hand, is one of the least popular worlds due to its sheer complexity, where virtually anything and everything can happen.
The complexity of Planescape derives from its construction. The Planescape multiverse consists of a number of different worlds called planes revolving around the central city of Sigil, a melting pot of the planes inhabitants. Everything, and everyone travels from plane to plane via portals - each one of which has its own key. Keys can be anything, from a drop of blood, to a stone, to an uttered phrase, or even a simple hand motion. The planes are subdivided into three main types: the high, neutral, and lower planes, which are further separated into good, neutral, and the chaotic. Whereas there is a mostly zealous authoritarian effort to uphold justice in the higher planes, the lower planes are the exact opposite no laws rule the land. The neutral planes occupy a middle ground somewhere between the higher and lower extremes. The good, neutral, and chaotic planes are exactly that. The higher good planes, for example, are full to the brim of radical law upholders, while the lower chaotic planes, lacking any sense of order, are at the polar end. These divisions are pretty clear and easy to see once you actually visit the planes.