Hard to describe.
His humor is very dry, sometimes to the point I have to read a book through a couple of times to catch all the puns and double entendres. Dry as it is, he is downright hilarious. The only author I like more than Pratchett is Mark Twain.
Nothing is sacred to Pratchett, he lampoons it all and it is always entertaining.
Magnificent satirist.
A good few first reads from Pratchett would be "Carpe Jugulum", "Feet of Clay", or "Witches Abroad", if you are inclined to check him out.