Borne through space, supported by four gigantic elephants standing on the back of a World Turtle (Chelys galactica), the Discworld is approximately 10,000 miles across. At its Rim it is approximately 30 miles thick, and it gets thicker towards the central Hub, where the massive mountains of Cori Celesti reach for the sky. A miniature sun orbits the Disc itself every day, as does a miniature moon, darting between the Elephants below and the sky above.
Yes, the Disc rotates. No, we don't know how the Elephants manage not to chafe. We assume that the Creator of the place made accomodations for that problem. We also assume that the Creator of the Disc took care of water recycling, for the oceans of the Disc constantly pour over the Rim's thirty-thousand mile circumferance in the spectacular "Rimfall", yet they haven't lowered any in recorded history.
The Disc is inhabited, of course. Mostly by humans in countries and cities of varying sizes, but there's also a sizeable dwarf (excellent miners with a thing for Gold) and troll (ambulatory bipedal rocks with not much brain) population, along with a scattering of various supernatural types (werewolves, vampires, ghosts, undead, etc) who manage to live alongside the normal folks. Inter-species prejudice is relatively unknown: Black and White have stopped fighting each other, and have teamed up to beat up on purple, green, and blue. However, -anybody- is tolerated in the major cities as long as they've got a decent amount of funds to back up their actions, so be sure to bring along a good supply of cash.
The usual laws of physics apply here: Things fall when released from up high, effect follows cause, etc. However, they are to be thought of more as guidelines, not as absolutes. In some places, the speed of light has been expressly -demonstrated- as being slower than sound. The sunlight doesn't just rise, it -pours-, and some places it can be captured and saved for a rainy day. There are also three major principles in effect, which help explain how the Discworld runs :
For the first principle, ANYTHING on the Discworld has the potential, in some way, to develop a form of life. It may not be truly 'life' as we know it, but that isn't going to be stopping the object in question from expressing its opinions on the matter. This is the main explanation behind the trolls, which are walking, thinking rocks; as well as the intelligence found in some wizard's staffs; or the just plain ornery attitiude in the case of a particular set of ambulatory sapient pearwood luggage. (Which was last seen wandering around the vicinity of Therin with no signs of stopping.) While it cannot be proven that every object in the Discworld has a viewpoint, it's often just safer to assume it.
The second principle is the fact that on the Discworld, Metaphor has a disturbing tendency to take itself seriously. Death as a robed skeleton is not just a metaphor for the end of life, he IS a robed skeleton with a full-fledged existance of his own. Belief is a potent force on the Discworld, and if enough people believe in a thing, then it -becomes- real. This is how the Gods of the Disc generally came into existance, because people believed in them. The powers of witches and wizards on the Disc also take advantage of this fact -- if enough people BELIEVE you can turn them into frogs, then you may never have to prove it. Also, things that are -not- believed to exist will never be seen; even if it's standing right next to you and waving its arms. One side-effect of this, however, is the tendency of the people of the Disc to take metaphors literally. Then again, given the tendency of metaphors to become real on the Disc, it's better to be safe than sorry.
The third and final principle in effect on the Discworld is the power of stories (quite possibly the most powerful of the three). It's not for nothing that we say that History repeats itself; the Marital Row, the Job Interview, the Rise and Fall of Empires, the Princess Marries the Prince, are all tried-and-true scenarios that people believe should happen in their proper sequence, and generally do. On the Discworld, to the right people, these cliches of time, these patterns are eminently recognizeable by those in the know. History Monks constantly monitor the goings-on to make SURE that history happens as it's 'supposed' to. Meanwhile, Wizards and Witches, with enough know-how, can 'derail' an existing storyline by presenting Destiny with a more attractive alternative. On the Discworld, the future is set. It's up to everybody else to fight back.
Due to the natural skew of Reality around the Discworld, magic is also possible, at higher levels in some ways than 'normal' parts of Shadow. However, as noted above, it isn't always practiced as regularly as would be expected. Why fling around spells and WORK for a living when public image and the trappings of wizardry and witch-hood suffice just as well? -Knowledge- is power, and they've got more than enough of it, so they don't need to regularly demonstrate it (though, in a pinch, they easily -can-... it helps keep down pretenders and riff-raff). There are also hazards to flinging heavy-calibre magic around... the fabric of Reality becomes especially weak around heavy concentrations of magic on the Discworld, allowing the potential of the Things from the Dungeon Dimensions to break through (either physically, or through the minds of those sufficiently engaged by an idea or magical power) and enter the 'Real' world. Whether they are Outsiders or just dwellers from another Shadow are unknown; what IS known is that they like and eat magic, hate all life and all living things, and that more than one incursion of them has been fought off in the past.
With that all said... the Discworld -can- be a fun place to vacation, as long as you take care to keep in mind that although there's high magic and oddball occurances, the people there are still living beings trying to get through life the best they can. Respect them, and they will respect you -- or at least not overcharge you severely.
Anybody wanna buy a sausage? Onna bun?