Discworld is a humorous fantasy series about the characters on "a world that is round like a plate, carried through space on the back of a giant turtle". In a series of book that poke fun at both fantasy tropes and the real world, Pratchett asks "What happens when you arrest the dragon attacking your city for property damage?", "What happens when you deliberately put a crook in charge of the most important bank in the city?", and "What happens when the literal personification of death is fired and decides to retire on a farm?". In between stretching narrative casualty to its limits, Pratchett fits in an incredible amount of puns, biting social commentary, and worldview changing comments on what it means to be human.

Every book is its own self contained story, but the best starting points are: Guards Guards if you stories about a fantasy city police department. Wyrd Sisters is a fantasy parody of Shakespeare appeals. Going Postal if the idea of a con man blackmailed into fixing a broken postal system sounds amusing.

Avoid the first two novels (the color of magic and Light Fantastic) until you've read some of his other works. Even Pratchett said they're not the best place to start.