Unearthed Arcana is a book full of variant rules. Arcana Unearthed (extended in Arcana Evolved) still isn't really a campaign setting, though it does include some details about The Diamond Throne, the "default" setting for AU/AE.

Read more at http://www.montecook.com/cgi-bin/page.cgi?arcanaevolved

There are only three official settings for D&D, though Wizards occasionally likes to publish a core rulebook for another setting before handing the bbaton to another comapny, These incluse the Oriental Adventures Rokugan setting which AEG is running and DRagonlance which was picked up by Bastion Press.

The official settings are Greyhawk (the default), Forgotten Realms, which shouldn't be new to anybody, and Eberron. Eberron adopts a strong pulp 1930s feel while staying true to D&D's fantasy dungeon-delving roots. Magic is advanced in such a way as to replace technology to an extent with continual flame lanterns and arcane mark-notarized banking cheques. Magic is far more common in the everyman's life, but the average level of NPCs if far lower. Four entirely new races were added, the Changelings, the Kalashtar(a psionic race, giving a way for psionics to be a lynchpin in an Eberron game if you choose), the Shifters, and the Warforged. Available to the core character races are Dragonmarks, which exist as feat that grant magical power to PCs and are passed down hereditary lines, such that merchant houses bearing these dragonmarks dominate the economy of Eberron. The final big difference is elemental binding, which is a way to cram an elemental into an object allowing for its power to cause a ship to sail faster, or a train-like vehicle to travel along the ground, or even to make an airship fly.

Perhaps the poster was confusing Unearthed Arcana with d20 Modern's Urban Arcana?

unearthed arcana like ghostwalk and some other rulebooks are variant settings. The variant rules included within alter the fundamental concepts of the game system and fictional atmosphere that they become a sort of campaign setting. they are like additional rules you can add to any campaign setting to make it unique. They arent complete campaign settings really at all adn do not include history of other realms or anything like that.

the most prominent campaign settings are:

Forgotten Realms- this is a standard dungeons and dragons world. it has a high level magic. its cultures are roughly similar to real world cultures. continents are roughly similar to the real world in shape and theme. Maztica for example is a continent to the west of Faerun that has jungles and primitive tribes etc.

Eberron- this is the newest official campaign setting and is very popular. it takes alot of elements common to anime and eastern cultures such as Super-dragon creator beings and pseudo technological/pseudomagical robots called the Warforged. I dont know that much about it but one thing is that the kingdoms are very large adn continents, which are separated from each other, are very culturally united.

Dragonlance- this campaign setting features dragons as a common place theme. Ancient wars between dragons etc. define the present. Dragon gods are have an important focus in the current setting. Magic is related to different colored robes which correspond to different High Towers. I believe there are three but I could be very wrong.

Darksun- is a discontinued campaign setting that might haev been the most original of the official ones. The whole world was ravaged by 'defilers'. Many people are ruled by sorcerous dragon kings. Metal is rare on the dessert planet. Psionics is heavily used. Themes include preservers vs. defilers. and class struggles.
There are a few more interesting races like in eberron... for instance the mul (sp?) is a half giant
this campaign setting was discontinued in 3.0 and more recent editions

Greyhawk- is the earliest campaign setting and is THE typical d&d world. Alot of mages like mordenkainen come from the greyhawk campaign setting. This setting deals alot with low-level adventures. and evil is more difficult to beat. Cities play an important thematic role in this CS.

Midnight- this is a very Tolkien-esque campaign setting where evil has control over the world basically. It focuses alot on very low level play. It is very difficult to kill even orcs in this campaign setting, partially bc evil doesnt allow people to haev weapons and wizards etc. are hunted down by powerful minions of evil. dwarves and elves are fleeting as in tolkien.

Relics & Rituals- this campaign setting focuses around ancient battles between titans and the gods. Remnants of this age effect the present such as left over monsters relics and armies.

Spelljammer- this is not wholly its own campaign setting as it can be added to any existing campaign setting. it features rules on creating magical spaceboats. Psionics plays a large role in this campaign setting. Different palyer races etc. come from different planets and spacetravel is not uncommon.

tehre are a couple others that i cant rememebr their names etc. most of htem were poorly thought of and boring.

forgotten realms and eberron are the most popular.

i forgot to mention Ravenloft!
ravenloft is also big... not sure quite what happend to it in 3rd E but it is a vampire undead world!
i think its on a moon or something. anyway it sucks pieces of other campaign settings liek forgotten realms adn dragonlance into it! in fact Ravenloft can be seen as a sort of undead alternate realty adn it is used this way by many gamers.