It is said that the Creator had a pepperoni and cheese pizza in mind when creating the Discworld, but slightly overdid it. The secret of pizza-making was given to Klatchian mystic Ronron "Revelation Joe" Shuwadhi by the Creator in a dream.
The original pizza can be found in the Forbidden City of Ee, although, of course, that would require finding that city first.
Pizza is eaten by student wizards who work until late on Hex, other humans, and more recently dwarfs and trolls. It has, however, been discovered that pizza should not be fed to swamp dragons. Known pizzas include the Quatre Rodenti (the dwarf choice), Klatchian Hots and Four Strata. Even Lord Vetinari has eaten a pizza at least once.
The mystical properties of pizza cannot be overstated. For example, note that the volume of a pizza of radius z and thickness a is pi times z squared times a.