The world is flat and round, kinda like a pancake, and rests on the shoulders of four elephants, who in turn stand on the back of a turtle (Great A'Tuin, if memory serves). This is not only scientific fact, but undisputable*. In this world, dwarfes and trolls exist, just as elves do (And they are truly to be feared, the buggers!) Teeming Metropolis Ankh-Morpork** bustles with life and crime, the former sometimes cut short by the latter***, the latter being fought by the Ankh-Morpork city watch, commandeered by commander Vimes, who is strongly supported by six-foot dwarf (adopted) Captain Carrot Ironfounderson (sp?), Angua the werewolf (who shares more than the grimy patrol routes with Carrot), the troll Detritus and the golem Dorfl, amongst many others.
At the same time, Witches cruise through the air, meddling in other peoples affairs, wizards try not to die from starvation in the Unseen University*****. One particular wizzard (spelled like this, yes) tries to outrun death, or rather Death, who has a great interest in him as his 'pet project'.
There's much more to be discovered. I suggest you get a few books (There's like a trillion of them already, because Terry Pratchet is a complete lunatic when it comes to output) and get to reading!
*Even though the sailors sailing too close to the rim of the Round Ocean tried to dispute it with a long, drawn out "NOOOOOOO" as they plummeted into the void, they didn't end up convincing anyone.
**Immortalized in shanties and songs like "Take me away from Ank-Morpork", "Ankh-Morpork, you ugly pimple" and "Oh, I wish I'd never come to Ankh-Morpork"
***This circumstance often leads to a meeting with Death****, WHO TALKS LIKE THIS.
****An anthropomorphic manifestation who owes his existance as a six-foot-high skeleton in a long, black robe to popular belief, that is, what the general subconsciousness believes to be true. He lives in the endless void beyond space and time, together with his butler Alfred and his horse Binky. He has a daughter named Susan, Who is a schoolteacher and just entered a torrid romance with the manifestation of Time.
*****Located one-third in Ankh-Morpork, one third in unknown dimensions, and two thirds in the architect's fevirish dreams, plus five-eigths in L-space******.
******Which is something only librarians have to know about*******.
*******The librarian of the Unseen University is an Orang-Utang who will screw your head off for calling him an animal.