"It is necessary to create constraints, in order to invent freely. In poetry the constraint can be imposed by meter, foot, rhyme, by what has been called the "verse according to the ear."... In fiction, the surrounding world provides the constraint. This has nothing to do with realism... A completely unreal world can be constructed, in which asses fly and princessesare restored to life by a kiss; but that world, purely possible and unrealistic, must exist according to structures defined at the outset (we have to know whether it is a world where a princess can be restored to life only by the kiss of a prince, or also by that of a witch, and whether the princess's kiss transforms only frogs into princes or also, for example, armadillos)."
Worldbuilding is the process of constructing a fictional universe. Strictly speaking, anything that happens in that universe "builds" it, so "worldbuilding" is only used to describe the invention of fictional details for some reason other than the convenience of a currently ongoing story.
A common form of worldbuilding is the creation of history. This could just be a Framing Device for a story told by a historian, but fantasy worlds regularly include historical notes for centuries of warfare and intrigue. Stories can then be written at various points along that timeline, and each of those stories will have a clear relationship to all the others. It makes the writing of serial fiction much easier, especially if the series has multiple authors. If so, the fictional universe is a Shared Universe.
The result may sometimes be called a Constructed World, conworld or sub-creation. The term world-building was popularized at science fiction writer's workshops during the 1970s. It connotes a focus on detail and consistency. Many post-The Lord of the Rings fantasy and post-DuneScience Fiction writers use world-building in an attempt to give their stories weight and meaning that they would not have without a well-defined setting.
Constructed worlds frequently have their own aesthetics, above and beyond the aesthetics of the stories taking place in those worlds. Some artists and hobbyists build fictional worlds with no intention of writing any stories in them—at least, none more detailed than historical documents.
Worldbuilding has two separate meanings:
The creation of a Fantasy World Map, history, geography, ecology, mythology, several different cultures in detail, and usually a set of "ground rules", metaphysical or otherwise. Sometimes, such worlds will have a Creation Myth that's either hinted at or told in more detailed fashion. This kind of worldbuilding can go to the extreme of working out entire constructed languages. Authors typically revise constructed worlds to complete a single work in a series.
The work that goes into deciding the details of a setting. It's very difficult to write a story that contains absolutely no imaginary elements beyond what's described to the reader, so nearly every author worldbuilds a little bit.
Over the years, this has explicitly become the goal of the creative team for Magic: The Gathering. Instead of being used to tell the story, each expansion block is now used to flesh out a different world to a remarkable degree.
The world of C'hou in With Strings Attached, a completely original world (which is a MAJOR rarity in Fan Fiction), fully realized, with two vastly different cultures and mindsets, several sets of slang, and hints of a much more ordered past.
And to a lesser extent, the Hunter's world, which the four visit in the Third Movement.
The authors of Game Theory have done a lot of world building for their version of the Magical Girl Lyrical Nanoha universe. They've expanded on the history and culture of the setting, the magic, the nature of alternate dimensions, and much, much more.
Hivefled has quite a bit of it for Alternia, most notably the subjugglator religion, Dyelus, which the author has written quite a lot of very detailed meta about on their blog, as well as the trolls' language.
J. R. R. Tolkien's Middle-Earth - The original, at least in the modern sense of the detail involved. Tolkien stated that the creation of Middle-earth was the result of giving his created languages a place to live in. He has written a lots of notes on the direction of that the history of Middle-Earth should go. Much of his notes have been organized and published as The History of Middle-Earth.
L. Frank Baum's Land of Oz (begun in 1900) is perhaps one of the earliest modern attempts at world building. Maps by Baum depict Oz's four regions and its neighboring kingdoms. The worldbuilding came about because of fans clamoring for more stories and places to explore. (Continuity Snarl ensued)
R. Scott Bakker's Eärwa (though not the entire planet), of the Second Apocalypse series, has four thousand years of human history, three huge religions, several different species, and his very own magic. Also, a completely incomprehensible Eldritch AbominationBig Bad.
David Weber's Honor Harrington started off less built, but after 17 doorstoppers plus 5 short story collections with several of the short stories designed to fill the history and technology roles have built up a fairly consistent world whose technology is plausible and has rather large and detailed conflicts.
Safehold exploits David Weber being a more experienced author, and has very large global conflict with dozens of different political groups involved in a religious conflict. The list of characters for David Weber series is comparable with that George RR Martin.
The Vorkosigan Saga, most notably in its description of Barrayaran history and culture.
Andrey Livadny's The History Of The Galaxy is a vast 'verse with over 30 novels, novellas, and short stories (and counting). The order of writing does not always match up with the universe timeline (although, currently, he seems to be primarily adding to the end). Most novels have unique characters, although there are story arcs that include several of the books. There's a reason the series includes the word "history", especially since many novels deal with alien races, most of which predate humanity by millions of years and some have stopped counting at billions. One novel even goes into the origin of life itself, and another off-handedly reveals that "true" origin of Christianity. Several fan-based web MMORPGs have been created based in the 'verse, especially set during the First Galactic War, a 30-year period of constant "technogenic" warfare between two human powers, a period so devastating that its effects are still felt 1000 years later.
Donald Kingsbury's Courtship Rite has a huge amount of world-building, not just in the planet Geta itself, but in the society of the Lost Colony, which has been there for so many centuries that Earth isn't even a legend any more.
Charlotte Brontë and her sisters and brother were masters of this at a very young age. Taking off in 1826 from a set of toy soldiers, they colonized the west coast of Africa and called it the Glasstown Confederacy. Then Emily and Anne split off and discovered Gondal, a huge continent in the Northern Pacific. It's easy to see the origins of Wuthering Heights and Jane Eyre in these narratives, which went on for their entire lives.
Henry Darger, author of The Story of the Vivian Girls in What is Known as the Realms of the Unreal devoted years, beginning in about 1919, to detailing the gigantic world where most of the story takes place.
Brian Jacques' Redwall books have a world that not only seems to span quite a lot of territory and varied geography, but also has a history going back many generations, including books set both before and after the original.
Worldbooks, a type of sourcebook, are tabletop RPG supplements that exist entirely to give Game Masters detailed settings to run their games in. While it's possible to buy worldbooks based on the real world—essentially, history books targeted at roleplayers—most worldbooks are about fictional worlds, and so the process of writing the book consisted entirely of worldbuilding.
There is even a "World Builder's Guidebook" giving a set of auxiliary rules for building one's own world in accordance with AD&D's mechanics. (A dice roll can result in the world being shaped as a twenty-sided polyhedron.)
There's an offshoot of roleplaying games developing that might be called "world-building games", in which the players collaborate to tell the history of a world that develops in-play.
Warhammer 40000 has an extremely well crafted setting with a complex history going back tens of thousands of years.
Warhammer doesn't have such an extensive time period to work with, but a higher number of factions and being limited to one planet means a lot more attention to small details.
Traveller deserves special mention as one of the best sci fi verses ever built.
Akira Tsuchiya's Ar tonelico world. To summarize, he created an extensive fictional musical language as a foundation for his world, complete with a physics section on how the language works in the world.
The Dragon Quest games are meant to be broken up into trilogies (1-3, 4-6, and 7-9.) However, there each trilogy bar the first only has a tenuous connection with the games they frame.
The Warcraft series has grown from a fairly standard setting to this, including lore elements dating back ten thousand years or more. It has four worlds (Azeroth, Draenor, and to a lesser extent, Argus and Xoroth) which are explored in depth.
Dwarf Fortress is something of a meta-example, as it does this the first time you play, and can be done as many times as desired. While the set of creatures, plants, and sentient races are well-defined in the game files, the mythology, history between the races, geography, and geology are procedurally generated, according to modifiable parameters. This is a huge part of the game's charm.
And of course, there are some games where you can build a world, or at least greatly influence one. Sim games are an obvious example. And the Fable games, amongst others, allow you to shape the future of the world they're set in.
The Nasu Verse often makes side-references to expand its magic system without it having any relevance for the actual story. The most prominent example would be the constant mention of dragons being the most powerful of all magical creatures, though nobody ever fights a dragon onscreen.
Marathon gets special credit for doing extensive world-building in a time when most FPS game stories consisted of "monsters teleport in, you kill them"
Halo started doing this with its extensive Expanded Universe, building an entire mythology around the series. The Anniversary re-release of the original game also has Terminals that give information expanding on the story to be told in the books, while those books to be written will be giving clues to the plot of Halo 4. Basically, it's like a plot cycle.
The Elder Scrolls has the Aurbis, the totality of existence, which encompasses Aetherius and Oblivion, and the Mundus (which contains Nirn, the world of mortals, and the continent of central focus, Tamriel), and a few other lesser planes of existence.
Touhou owes its gargantuan fanbase to this trope. ZUN includes a surprising amount of information surrounding the characters, events and setting of the games, enough to fill several Universe Compendiums, but it is almost always bare-bones details, prompting the fans to create their own world building in any way they can, filling whatever gaps they see.
The Otherworld Project, formerly Eshraval, is a long-running online collaborative modern worldbuilding project founded in 2004, which also encourages Role Play in the context of the world (though not at the moment since it's rebuilding). It's recently undergone a reset, and is in redevelopment mode. Strong hints of Crapsack World in its current incarnation.
The Land of Ooo from Adventure Time is a mild example. Ooo is meant to be After the End, arising from the remains of human civilization as we understand it, and there are hints towards this in almost every episode, including the opening sequence. Examples include mispronunciations of names like "Mozart" and "Groucho," and there's also a Nursery Rhyme that seems to be about mushroom clouds.
In the early seasons of The Simpsons Springfield semi-qualified, as its events were contained enough to function as a separate world, even if it was never defined as such. This was abandoned later on.
Thunder Cats 2011 implied a lot about not just Third Earth, but the entire universe, but never got around to revealing much. The animal races that were brought there after the Black Pyramid crashed have been there for about five thousand years, and have had conflicts between themselves before the start of the series. There is also mentioned other inhabited planets, but we only see one during a flashback.
The Art Of Kung Fu Panda goes into some of the history of the world, such as how Master Oogway came to the Valley Of Peace and why most of its residents are pigs, geese, and rabbits. Both this and the animated shorts also show off a few important locations that never appeared in the films.
It's rather easy to do this yourself, just open up a word processor document and let your imagination take off.
Santharia is a world-building project for the world of Caelereth, which has been going since 1998. Everything from flora and fauna to cosmology is described in loving detail, and pictures added created by Dreamers themselves. The world of Caelereth is developed on the Development board, while on a separate Roleplaying board stories are told set within this world, most of them within the continent of Sarvonia. Recently an interactive game has been developed.