I'm a fantasy writer, and fantasy novels take place in fantasy worlds, so I thought I'd discuss the two methods I've used for building worlds: mapping and concept exploration. (Actually, this is a bit of a cheat post, because I wrote most of it for thread I started on a forum, and I liked it enough I decided to spread it around.)
I find mapping to be an excellent way to get a fantasy world started if the world is not dependent on a fundamentally unusual concept. If I intend to have a quasi-medieval setting with magic and monsters and a variety of humanoid races, mapping is a good place to start. It doesn't even tax my limited artistic talents, because the outlines of continents don't need to look like anything in particular. In fact, if I just scrawl an irregular line, I often see a shape in it the same way you see shapes in clouds, and those Rorschach reactions may inspire geographic names: "Devil's Head Coast" or "The Unicorn Peninsula." (I'm not holding those up as good names, mind you.)
I generally start with a main continent, throw down some mountain ranges (using ^ marks or just a broad line), and then get to naming places. "The Anvil River," "Plains of Wrack," "The Glittering Sea." Then I start asking myself questions. Why is the sea "glittering?" Maybe it's a very shallow sea, but tossed by violent storms and chock-full of sea monsters, so there are tons of shipwrecks at the bottom, and on clear days (when you're not busy fighting off ocean beasts) you can see glints of treasure beckoning from the bottom.
In answering these questions, I find I open up other questions. If the sea is so dangerous, why is there so much traffic across it? Perhaps there's another continent on the other side where gold and jewels are plentiful, and the natives don't even see any value in them. So now I draw the new continent and have to answer questions like, "Are the natives of this rich continent civilized and powerful, backward and savage, or just unconcerned with worldly things, perhaps a woodland society of satyrs and nymphs?" And, "Have the nations of the main continent ever tried to conquer the mineral-rich one?"
The answers to all these questions get me started on political divisions and history (maybe those "Plains of Wrack" are the devastated ruins of a vast city, blasted to wreckage in some long-ago war or catastrophe), and I make notes of all of that as I continue to draw new geographic features, mark the boundaries of countries, dot in important cities, and so on.
My other method involves planning a world around some big idea, some concept that makes it significantly different from other worlds I've read about. For instance, while I've read plenty of s.f. novels about water worlds, I've never read a fantasy book set on a world without land. Let's say I want to do an ocean-based fantasy world. In this case, I have to jump straight into asking about the ramifications. If there's truly no land, where do people get building materials from? Perhaps more importantly, are there surface-dwelling people at all, or are all the races in this world aquatic? I don't want to get too alien, so I go with surface dwellers. But surface dwellers with arms and legs and feet imply one of two things: either they are immigrants from another world/plane of existence, or they are the remnants of a time when land existed on this world. Each of those possibilities carries with it an important historical event: the arrival of the first immigrants, or the disaster that somehow flooded the surface of this world with so much water.
I like the sound of a disaster, but I also like the idea of an intrusion from another plane, so I decide to put them together. At some point in the past, a powerful mage opened a magical gate to the bottom of an ocean on another plane. Perhaps he was experimenting and something went wrong, or perhaps he was trying to irrigate a desert. In any event, he could not close the gate, and eventually, the water level of the two worlds equalized at a level that drowned all the continents of the wizard's world.
Now there are lots of other questions to answer. How long did the flooding take? Is it possible people weren't even worried at first, especially if a new river flowing from the gate helped them turn a desert into fertile farmland? What else came through the gate along with the water? What steps did the nations of the flooding world take, once they realized that sea level had begun to rise? What ocean-dwelling races were already present in the world -- mermaids, sirens, sea dragons? How did those creatures respond to having their territories enlarged, but also to having the balance of their ecosystems upset?
Just by posing and answering these kinds of questions I can quickly come up with a wealth of background material and story ideas, all of which flow organically from the original concept. Then it's just a matter of fine-tuning them and deciding on the specific story I want to tell within the setting.
At any rate, those are my two main methods. I usually use them in some combination, either starting with a map and later working in concepts that make the world fundamentally unique, or starting with the concepts and creating a map once I feel I have the conceptual ideas well established.
For my Delvonian Empire setting, I started with an idea I'd had back in high school, of a world with floating continents suspended in air. I'd expected it to be a science fiction setting when I first imagined it, but by the time I returned to the concept, I'd changed my emphasis to fantasy (and found the science of floating continents to be awfully suspect anyway). To start with, I used the conceptual method, asking why the continents were floating in empty space. My answer was that a war between the gods ripped existing planetary worlds apart, nearly killing all the gods' worshippers and causing them to realize that acting within the mortal plane was maybe a bad idea. This led me to list out a bunch of deities for the world and construct an explanation of how the gods had reorganized the torn-apart matter of the planets in order to save the surviving inhabitants. Readers of the books will notice that almost none of this has made it into the books. So far, I haven't developed any stories that require the reader to understand this part of the world's backstory, so rather than squeeze it in artificially, I've just left it unsaid.
Once I had the background ready, and explanations for the major phenomena (like floatstone as the reason continents remain suspended), I turned to mapping. I started off with a large-scale map of Delvonia Major and Delvonia Minor, Hiisia, and a couple of minor continents. I drew these twice, once showing the major political divisions before the rise of the empire, and again showing the borders as they exist in the present. I did it that way because I knew that the empire was comparatively recent, and I wanted the feel of its various provinces to be that of independent cultural entities.
Okay, enough for now! If anyone finds this interesting, let me know, and I'll return to the subject in some future post.