This is a unique world with its own structure. The planes are configured differently, although they are still compatible with D20 fantasy games such as D&D and pathfinder - spells and other abilities still work in the way you expect. It is just the underlying principles that change. There are also unique rules that control how the gods and pantheons work - again on the surface everything is pretty much the same - but there is a bit more structure behind things.

These are the rules that underpin the game world - most of them deal with structural things, and players don't need to know how they work. Come to that GMs don't really need to know how they work either :) Although it might prove useful to know.

The Universe