His best known books revolve around Discworld, a world controlled by numerous gods, where magic and narrative causality take the place of physics, where a million-to-one chance is always a sure thing (but not a million-and-one-to-one chance, because that'd just be silly). Discworld is supported in space on the backs of four elephants, which in turn stand on the back of the Great A'Tuin, a gigantic turtle swimming through the vastness of space. Starting with The Colour of Magic, the series features such characters as a scythe-carrying Death and a deadly piece of luggage with legs. Unseen University - named after the Invisible College, the forerunner to the Royal Society - is where wizards receive their training in magic, or specifically, how to not use magic, as Pratchett himself found the concept fundamentally boring so took a more satirical stance on the traditional world of spells and sorcery.
His earlier works, such as The Colour of Magic and The Light Fantastic were straight parodies of the fantasy genre. However, as his writing matured he began to parody other genres, within a fantasy setting; these included: mysteries, crime fiction, political fiction (The City Watch series), horror (Reaper Man), sports fiction (Unseen Academicals), action/adventure novels (The Last Continent, Reaper Man), westerns and a little bit of hacker fiction (Going Postal). More importantly, his novels changed from being largely parodies to being satires as well, taking on a wide variety of subjects with incisive wit and humour. Topics touched upon in his later works included rock music and the price of fame, "headology" (folk psychology), "lies-to-children", technological development, engineering, startups, war, diplomacy, the essential con trick behind fractional reserve banking, the operational independence of the police and the judiciary, corporate corruption... all without being the least bit boring or heavy-handed. The number of hidden allusions in his works to other cultural works and real-world events is remarkable.
It is not impossible to say that the Discworld series was one of the influences absorbed by the author of the popular Harry Potter series, and may well be one of the most influential fictional series of all time. At least the most influential without a multi-million dollar movie franchise based on it (there are a few made-for-TV adaptations and cartoons, however), which may or may not be a good thing. One of the earlier Discworld books centered around a young child entering into a school for wizards, but Pratchett didn't think there was a future in basing a book series around a wizard boarding school.
Among his other notable works is the series of popular science books The Science of Discworld, written in collaboration with Ian Stewart and Jack Cohen. Despite the apparent similarity to things such as "the science of Star Trek", the books had very little to do with the science of Discworld and instead the science of the real world. In a fairly ingenuous concept, the series explained common scientific principles by contrasting the real world, which runs on rules, with Discworld, which runs on narrative principles and supposed common sense.
Mr. Pratchett also wrote, along with Neil Gaiman, the only nice and accurate description of the coming Apocalypse under the title Good Omens. To date, Mr. Pratchett is the only author to have spoken directly with Death, and on further examination has deemed him to be a sensible fellow, if not with a dark sense of humour.
Terry Pratchett was also a figurehead of a campaign to save the orangutan. The librarian of the Unseen University is a wizard who was changed into an orangutan in one of the accidents that frequently take place among the careless and somewhat incompetent university wizards. Uniquely, the Librarian has no wish to be returned to human form and has gone to considerable lengths to ensure that such can't happen.
News was released on 12 December 2007 that Terry Pratchett was suffering from Early Onset Alzheimers. Following his diagnosis, he made notable monetary donations and worked to aid Alzheimers research. He also became an outspoken supporter of assisted dying, giving an impassioned speech on the subject (albeit via his friend Tony Robinson) entitled "Shaking Hands With Death" for the 2010 Richard Dimbleby Lecture. Following this, he also worked on a documentary about assisted suicide for the BBC entitled Choosing to Die that televised the death of a man who took his own life in the Swiss Dignitas clinic. The film attracted some manufactroversy from opponents of assisted suicide, claiming that televising both the documentary and the lecture amounted to extreme pro-suicide bias on behalf of the BBC.
- On one of his Usenet postings, Terry Pratchett mused that not before the computer video game Doom did anyone ever think of the shotgun as an appropriate way to stop a demon.
- He received an Order of the British Empire (Commander) on New Year's Day, 2009. He commemorated the occasion by forging his own sword out of meteorite iron.
- Following his death, fans created code to keep Terry Pratchett's name alive on the internet, emulating a plot device in Going Postal. 
- Good Reads - Terry Pratchett quotes.
- The Guardian - Terry Pratchett: my case for a euthanasia tribunal
- 2010 Dimbleby Lecture on YouTube (If you don't laugh and cry while watching this you are dead inside)
- BBC News - Sir Terry Pratchett unswayed by assisted suicide film
- Daily Mail - Peers in attack on BBC over Dignitas death as they accuse bosses of campaigning to change law on assisted suicide
- Just how freakin' cool is that, huh?
- BBC Interview with Mike Lawson
- GNU Terry Pratchett "A man is not dead while his name is still spoken."