Standing on the surface of the world, it doesn't look so different from Earth- after all, it's still got mountains, blue sky, dark clouds, stars, a moon, and the sun. There are birds and insects and grass and trees, rivers and lakes and streams. It rains. It snows. The wind blows and sometimes it doesn't blow.
But if you look closely at the sky, you'll notice that you can see currents in the sky, and that the light from the sun is diffused- you can't ever see the actual star. And the fact that the sky (clouds and all) are moving in an obvious counter-clockwise pattern is pretty suspect. Now that you look at it, it looks a damn lot like they're swirling around a midpoint like a hurricane is forming.
But it's not.
The world is made up of a series of rotating planes, one above another. The lowest one, where you're standing, is fairly normal once you get past the way the sky looks, and the fact that you really can fall off the edge of the world. It doesn't even rotate, unlike every other plane.
The next layer up is the Astral Sea, made of a silvery water. If you looked up from the Base you wouldn't see that there are substantial floating islands, but there are, gently drifting with the current and forming archipelagos and formations in the sea. Creatures native to the Astral Sea tend to be enormous carnivorous fish, krakens, and other dangerous semi-mythical beasts. There are often bizarre plants and small animals on the islands in the Astral, completing a very basic cycle of life. Holding up this whole mess is the Firmament, a relatively thin, transparent, gooey, self-maintaining membrane. Piercing this membrane is what causes waterfall or, if it's cold enough, snow.
Above that is the Darkness, a semicircle of what appears to be an impossibly strong sort of hazy glass, and floats exactly where it's put. The day-night cycle on the Base plane is caused by differing degrees in the thickness of the Darkness. At its thickest point, the Darkness extends vertically for miles- at its thinnest, it is as a rime of ice on a lake in the early spring. This is where the Gods live, parcelling out territory for their servants, and concerned only with eternal war. The Darkness's top is bathed in nearly constant sunshine, except where the Gods will it dimmed for their bizarre and seemingly petty motives. Gods live only on the thickest portions of the Darkness, which they have nearly absolute control over, leaving the thinner portions to lesser Gods or even particularly powerful demons. The variety of the plane is endless and, because the Darkness is such a truly massive plane, nearly anything can be seen on its top.
The last layer, the Light, is inestimable. No mortals have ever managed to reach so far as the Light, and not even the Gods seem to know what's going on (although they're not particularly talkative to mortals.) Some say that it's an enormous glowing orb, perhaps the face of an eternal overgod who watches and judges. Some say that it's the end of the universe itself, and that without the influence of the Darkness over them, we'd all be cooked alive. Some others say that it simply is, and one might as well ask what a mountain is.