Basically, at some point in the past, the multiverse of planes the once existed, broke apart, shattered. The Gods of that era were killing each other and wrecking pretty much everything at the time, and are credited for the causing the planes to fracture and splinter off. What's left are a jambalaya mix of planar shards: small fragments of a shattered planescape. Some planar shard are still inhabitable.
The game would start several thousand years after the cataclysmic event. In that time technology has advanced enough to accommodate interplanar travel on an enterprise level.
The only way to travel from one plane shard to another is by using a special ferry service where ships are powered by elementals and protected by a complex array of protective barriers. However, the void-space between shards is pretty hectic with very weird phenomena all throughout; shipwrecks aren't uncommon.
It boils down to this: you guys, some normal (or abnormal) joes get marooned on a shard of the <whatever-plane-I-decide-on>. Lucky you, right? Unfortunately the elementals bound to the ship were freed in the crash, the ship itself is totaled, and three of the eight wizard-guys that powered the dimensional barriers are dead. It's a survival game where you need to find the missing components for your ship and escape from this extremely hostile environment alive.
There's more plot after the initial survival-horror tidbit, but I won't get into it just yet.
Plane Shift and other teleportation spells can be cast normally, but you may accidentally end up inside the voidspace between planes, if you travel to far and immediately cease to exist.
The planar shards are always moving around like little white flecks swirling around inside a constantly shaking snowglobe. Plane shards exhibit physical properties similar to those of massive objects existing in the material plane.
Plane shards produce quasi-gravitational effects, thus they interact as stellar bodies would. These effects keep plane shards from colliding with one another, yet still elicit some semblance of organization out of the chaos; for example, small satellite bodies (planets) would orbit a much larger, super-massive object (e.g. a star). The larger the shard, the stronger such effects are.
Then there are the guilds... *sigh*
I'll put it this way, traditional kingdoms and many religious organizations weren't able to survive after the shattering of the planes. It was the trading companies and guilds that picked up the slack. After the transplanar tunneling technology was developed and kickstarted the ferry industry, the heavily decentralized landscape of governing organizations (mostly guilds or worker's unions) were quickly network together. After a series of bloody conflicts, came the rise of bureaucracies and the policing entity that would come to govern all the different organizations: Meta Prime.
Pretty much every creature in existence is part of a guild, union, or similar organization. And... pretty much every "guild, union, or similar organization" answers to Meta Prime. As a result, you have the "Inner Realms" and the "Outer Realms". The Inner Realms are places where Meta Prime has influence and the Outer Realms (I like to call them "the boonies") refers to the far edges of the planescape where Meta Prime has little to no influence. And, it's not a matter of insufficient resources or personnel; it's a matter of insufficient motivation (i.e. No one cares about the friggin' boonies! xD).