Oerth = home of the World of Greyhawk game setting and timeline we all know.
Earth = This is our world. It is also the home world of the mace wielding deity St. Cuthbert and the technology of Murlynd.
Aerth = Essentially the setting of Gary Gygax's Dangerous Journeys RPG and books.
Yarth = Is the home of Gygax's Gord novels, the Sagard the barbarian stories and quite possibly the original home of Mayaheine before she was brought to Oerth by Pelor.
and finally there is Uerth = The "gothic" world. This is a place of pulp horror fiction and could possibly be the location of Rhop a land from which the nomadic Rhennee left for Oerth.
Rip's theories are well reasoned and authoritative in my opinion. However, in published canon we have learned Uerth is far worse than just an HP Lovecraft world; it's the EVIL MIRROR UNIVERSE OERTH. This development came 10 years ago, in 3.E's Expedition to the Ruins of Greyhawk by Paizo Publishing's own Erik Mona, Jason Bulmahn and James Jacobs in what was basically the last major Greyhawk adventure ever printed. What follows is for DM's only. It contains major ***SPOILER*** material. You've been warned, ready?
Still with me?
Okay then. In the adventure there is an artifact called the Orb of Opposition. Touching it transports a character's evil duplicate to Oerth. Enough said. Without giving too much more away, the following NPCs have mirror opposites floating around:
Lord Robilar's evil doppleganger is thus Bilarro.
The utterly evil Iggwilv (Tasha) is replaced by the "good" Ahsat.
Mad Zagig Yragerne has a double named Xagig.
Nolzur has a copy called Rulzon.
Quaal is mimicked by the evil Aluuq.
Keoghtom's evil version is Komoghet.
Musical Heward is opposed by the evil Wedrah.
and Murlynd has the evil double named Lyndrum.
All quite anagramatically funny yes, that is a classic Gygax hallmark, but making Uerth a literally intrinsic evil realm opens up VAST possibilities for a campaign that wishes to gate to and from this world. For example I could do this:
Iuz the Old One, demigod of evil and pain becomes Zui the Young, demigod of good and life. A being whose embattled land is a holy island surrounded by a sea of abject evil. (I'd say that anything higher than a demigod or demonlord is unaffected by the mirror world effect, so Nerull is still Nerull and Pelor is still Pelor.)
Celene, Kingdom of the Elves ruled by Queen Yolande becomes, Celune, Realm of the Dark Elves ruled by Queen Andeyol.
The good Kingdom of Furyondy is now Furyon ruled by the anti-paladin King Vorleb.
The wicked Great Kingdom becomes the once Grand Kingdom of Uerdy -now fractured and overrun by millions of peasant zombies, while undead-slayers and paladins like Saint Gothkar roam the land defending the weak.
Adventure hot spot the Tomb of Horrors becomes the Crypt of Terror a place created by the mage Karereca, that openly challenges evil, greedy treasure seekers to risk their lives for fame and fortune.
The Temple of Elemental Evil becomes the Temple of Elemental Good (of course) where an angelic ally of Zui is bound after her defeat in the Battle of Meridy Fields.
The Free City of Greyhawk ruled by wealthy neutral oligarchs becomes ,well, the Free City of Grayhawk. I imagine the balance is important after all since this place maintains its neutral stance, albeit attracting a more vile sort of adventuring lot.
Irongate the city fortress of good humans and dwarf-kind becomes Ironmaw a city of evil inhabited by vile humans, drow, duergar, hateful gnomes and yes, cannibal halflings- their city is only part of a larger faction called the Iron Horde.
The noble Kingdom of Keoland is now the Kingdom of Keoghlund a vast empire whose spy network, the Knights of the Watch cause fear and mistrust in all citizens.
The nautical Hold of the Sea Princes here, becomes the Domain of the Sea Princess. All must fear this narcissistic princess and her deadly pirate fleet!
Finally, the tiny, brutal land of Ull on Uerth becomes the paradise Luu where peaceful herders mingle with unicorns, gentle giants and other fey denizens. BLECH!
I could go on like this for multiple posts and never run out of things to adapt. And yeah, I know Oerth is already a world of ascendant evil, but like I said Uerth can be this twisted mirror-version of the World of Greyhawk that turns the evil dial to 11. Good luck Greyhawk fans, I hope you take this ball and run with it now!