Social Commentary
Terry Pratchett makes excellent use of Horatian satire. He has created a world in which we can see glimpses of our own: for example, the largest city on the Disc is Ankh-Morpork, whose citizens speak with an accent that is remarkably close to that of the average Londoner. We chuckle rather than wince at the way he points out some of the faults in our society. However, the Disc is rather small, and so it magnifies the foibles of human existence. Pratchett writes in such a manner that we laugh out loud, then stop and think "Hey wait, he's got a point..." In "The Fifth Elephant" Vimes comments on the fact that people are willing to blindly follow tradition.
"You did something because it had always been done, and the explanation was "but we've always done it this way." A million dead people can't have been wrong, can they?" --(Terry Pratchett, The Fifth Elephant)
The book "The Fifth Elephant" is a look a colonialism. Uberwald, as the place in question is called, has just come into the world view because of the economic possibilities. There are 'fat deposits' in Uberwald that the rest of the Disc would like to get their hands on. Referred to as 'the Dark Continent,' Uberwald bears a striking resemblance to Africa.
Through the characters of the Discworld it is much easier to see the flaws in our society. By setting his books on a different world, Pratchett allows us to step back and take an objective look at the way we do things. Employing humor makes the problems he points out much easier to accept. However, reading these books and seeing the similarity between the way they do things on a flat world carried on the back of a space turtle, which is on the very edge of reality, according to its own wizards, and the way we do things here on Earth it makes you wonder...
Pratchett also makes various references to other works of literature. For instance, in the book "Moving Pictures" Pratchett uses the line
"No-one would have believed, in the final years of the Century of the Fruitbat, that Discworld affairs were being watched keenly and impatiently by intelligences greater than Man's, or at least much nastier; that their affairs were being scrutinised and studied as a man with a three-day appetite might study the All-You-Can-Gobble-For-A-Dollar menu outside Harga's House of Ribs..." -- (Terry Pratchett, Moving Pictures)
Woah, I want to know about more references in his books! I want to go to L-Space