Skyslands (pronounceds "sky-lands") are a difficult matter to explain.  This is the best way possible.
The world is much bigger than we think.  Earth is actually flat, seven continents suspended in an ocean.  When one reaches one side of this ocean, a magic screen known as "the Mist" transports them to the opposite side of Earth, giving the illusion of sphericality to the world.
"If Earth is flat, how can it be a planet?"  one would argue.  Earth is not a planet.  Earth is a skysland (explained later).  There are planets, spheres of stone floating above the world.  Most of these planets are dead, cratered rocks whose life forms died long ago.
Now, onto the skyslands.  Skyslands are entire worlds, land masses and oceans anchored into cuplike pillars of stone.  These stone pillars are indescribably high, reaching up into the sky for miles and miles.  The bottoms of the skyslands are carried on the backs of world turtles (Atlas giganticus).  These gargantuan sea-turtle-like animals swim in the Endless Sea, a body of water that seems to stretch on forever.  The origins of the world turtles are unknown, as any fossils that may or may not be of their ancestors would have sank into the bottomless ocean a long time past.  Some think that they came from the planets, taking life with them on their backs.
There are literally millions of skyslands, each on the back of a world turtle, and each teeming with life.  One would think there would be more in an infinite ocean, but the Endless Sea is not infinite - it does end, with a wall of cliffs that drop down to unknown lands.  Some believe that the Endless Sea and all its skyslands are simply another, larger skysland on the back of an even larger world turtle, in another skysland held up by another world turtle, and so on and so forth ad infinitum.
Most skyslands have no Mist, being open to the rest of the worlds.  Earth, however, is not.  Long ago, humans did travel between skyslands, on the backs of fantastic beasts and in special "skyships" used by all other civilized realms at this point.  However, the humans eventually gained too much power, and became evil tyrants upon the other skysland inhabitants.  They were summarily rebelled against, driven back to Earth, and imprisoned there by use of the Mist.  Today, no human has any memory of the skyslands ever existing.  Whether this is a good thing or not is unknown; humans have, in their isolation, developed technology, largely unused by other skyslanders for magic being there.  Humans may one day use technology to escape Earth, and who knows what will happen to the skyslanders on that day...