In my years of reading fantasy, I've come across five different types of world: the created world completely unrelated to Earth, the alternate Earth, the far future Earth, the future colony planet with different rules of physics than Earth, and the ancient historical Earth.
In the first camp, we have the world of David Eddings' Belgariad as a popular example. This world, at least as far as I know (I haven't read beyond the Mallorean), stands on its own. It may or may not exist in our universe, and really it doesn't matter: the world was created by the seven gods, and is sufficient unto itself as a story setting.
In the second camp, we have alternate histories set in medieval or similar time periods of our own Earth, with added fantasy elements such as monsters and magic. The Once and Future King, T.H. White's story of king Arthur, is a good example of this. In fact, any retelling of King Arthur, or of the Norse hero Siegfried, would fall into this description.
In the third realm you will find stories such as Jack Vance's Dying Earth. These stories take place on our Earth, millions of years in the future when the sun is red with age and near to burning out, the Earth has gone through multiple cataclysmic upheavals, the human race is all but extinct, and magic is both real and powerful.
In the fourth type of world, you will find Anne McCaffrey's Pern (and the Dragons, and Dragon Riders, therein). It may be so far into the future that the human generations now dwelling there no longer remember that they were once a colony of Earth, but it is still a future and far off colony planet.
In the last realm, you will find J.R.R. Tolkien's Middle Earth and Robert E. Howard's world of Conan the Barbarian. both of these worlds are set in a fictional ancient past of our Earth, when magic and monsters were real. Presumably, either the creatures of magic all departed, or some undefined change took place which left us with the mundane world we have today.
So what is the world of Aerde? Well, if you've read the byline of this blog, you may already have guessed: I am following the path taken by Tolkien and Howard, and setting the world of Aerde in our own Earth, thousands of years in the past. I'll talk more about why I made this choice in another post.