Roundworld is a sphere-shaped world. Yes, it is silly, but people seem to be able to stand on it, so we shouldn't worry too much about it. It lies in a universe created by the wizards of Unseen University, as a way to use up the excessive magical energy generated by the splitting of the thaum. (However it should be noted that travel to the Roundworld and references was possible before the wizard's creation so it may not have happened yet.) Hex watches over the Roundworld, and can move small things in it and influence it slightly. Apart from that, it is (in theory) completely isolated from other universes, and there's no magic in it, not even essential elements like narrativium. There aren't even any gods. After several failed attempts, the wizards managed to create a nice planet (although it's not plane at all, which is quite depressing) where life started to develop.
For all intents and purposes, Roundworld is "our" planet Earth - it is home to a human civilisation which mirrors our own, though several other civilisations have risen and fallen in Roundworld's past, including the crabs and dinosaurs. Roundworld's human inhabitants have not led an untroubled existence; their history was severely threatened by an infestation of Elves, who twisted the stories of Roundworld humans (notably William Shakespeare) to their own ends, and later by the Auditors of Reality, who objected to the consequences of Charles Darwin's Origin of Species. The Wizards acted in both cases to protect their preferred Roundworld history; they feel a certain level of responsibility for the entire universe they have created, and especially for the humans who dwell in it.