From Discworld & Terry Pratchett Wiki
Throughout this wiki, there are countless references to Roundworld counterparts to Discworld characters, events, phrases, locations, norms, periods &c. The name "Roundworld" is one coined by the Wizards and is used in this wiki to show when Terry Pratchett has taken a real-life example from Earth (the world in question, which is indisputably round) and twist it into a Discworldian parody. Examples include the Diet of Bugs (read "Diet of Worms"), the Piecemaker (read "Peacemaker"), Old Tom (read "Great Tom"), Leonard of Quirm (read "Leonardo da Vinci") and literally thousands of others. There are longer lists in the articles on the individual book as well as Annotations pages.
Roundworld as a place
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.
- It is implied in The Science of Discworld IV: Judgment Day that the crab civilisation may have been distantly remembered, possibly in genetic memory, by later civilisations on Roundworld. No doubt emboldened by his rescue of the Ephebian library's contents in Small Gods, the Librarian nipped into Roundworld (probably via the medium of L-Space) and rescued the contents of the Library of Atlantis prior to its destruction, bringing them back for posterity at the University Library and later enlisting specialist help to decode the language. The only problem is, the wisdom of Atlantis was inscribed by skilled claws and mandibles in clay tablets by what is described as "a lobster-like civilisation". So. Atlantis existed. But not a human one....