Planescape is a series of novels, an RPG
and a computer game
based on the concept of the Multiverse
, the idea of multiple universes
, and owes a lot to the work of Michael Moorcock
Planescape has a strong focus on personal philosophy. It's central focus is that belief creates reality in a very real way, a fact that forces the players to consider the philosophical and religious stances of their characters beyond the "he likes bashing things but he's a nice bloke realy...he's Chaotic Good" level, something that few other RPGs successfuly do (respectful nod to Mage the Ascension).
As a direct result of the very physicly important nature of Belief in the Outer Planes the central organisations that the PCs are forced to deal with are not theives guilds or Megacorps, but what are essentially guilds of philosophers; organisations which assist and promote those with similar views on the nature of the multiverse. Descriptions of the Fations as they are called can be found here. A glossary of Planescape terms and slang is available here.