Simply put, Discworld is an imaginary world that looks like a disc carried on the backs of four elephants, which in turn rest on the pockmarked shell of a giant turtle called the Great A'Tuin. It is an invention of the famous science fiction writer and satirist is Terry Pratchett.
Discworld, as one can guess, is the setting for many imaginative tales. In fact, there are more than 30 novels set in the Discworld universe to date. It is quite a complex world, with its own set of political ideologies, cultures, paradigms, and general rules of physics. It is also filled with magic, wizards, witches, gods, goddeses, gargoyles, politicians, and an ethnically diverse police force called the City Watch which has had its fair share of various strange creatures such as trolls, zombies, vampires who don't drink blood, werewolves who are mostly vegetarian, and six-foot dwarves (adopted).
While each book in the series can stand on its own, most fanatics would advise the new reader to read the books chronologically if possible, as the books happen in real time, and the characters age and develop as the series progresses. It should also be stated that the Discworld series tells the stories of a diverse group of Discworld citizens, some of whom don't even know that the other citizens exist. The heroes and heroines of various novels come from disparate cities, countries, and continents, each with flavors and textures of their own.
Most enthusiasts agree that Discworld as a series might be better understood if it were divided into its subsets: books that involve Rincewind and the Wizards, books that involve the Witches, books that center on the City Watch, the Tiffany Aching books — a Young Adult subset — and the others — a subset that includes all the books that aren't included in the sets mentioned above. Some readers simply pick one aspect or subset that they like — the City Watch, for instance — and only read books that focus on that plot arch, arguing that the scope of the series is just too broad to embrace in its entirety.
Finally, it should be noted that the series has and still is enjoying immense success, especially in the UK. The novels often top bestseller lists; in fact, Terry Pratchett was the UK's bestselling author in the 90's, an honor due in huge part to the success of the Discworld books. Pratchett has also announced plans for at least 2 more Discworld novels.