Neil Gaiman and Terry Pratchett are both famous on their own - Gaiman best known for The Sandman (also known for many other things), and Pratchett for the Discworld series of books. Both are incredibly inventive, and can be very funny. Their teaming up to do a fantasy novel about the apocalypse is an ideal combination.
The book starts with the arrival of the Antichrist on earth, where Satanists attempt to perform a baby swap - placing the Antichrist in an unhappy home where he'll grow up evil. But they're kind of part-time Satanists and the swap goes awry, so the Antichrist grows up in a rather normal British home. As he grows up, the angel Aziraphale ("An angel, and part-time rare book dealer") and his buddy the demon Crowley ("An Angel who did not so much Fall as Saunter Vaguely Downwards") watch over things. Aziraphale and Crowley are technically from opposite sides of the fight, but they've both been on Earth for 6000 years and have got kind of used to each other.
This is one of the funniest books ever written ... with the caveat that you have to like absurdist fantasy and be okay with them happily shredding Christian mythology. If you like either of the authors, you'll have a pretty good idea what you're in for. One of the things that fascinated me about it was how well the two of them writing together worked: they fill out each other's weaknesses and build up the other's strengths, producing lovely prose and howlingly funny jokes. A blurb on the cover from the New York Times declares this "A direct descendant of The Hitchhiker's Guide to the Galaxy," and that's a very accurate assessment: this is to religious fantasy what HHGttG was to science fiction. Since Pratchett's death, there's no hope of him and Gaiman doing it again, but I'm glad they did it once: it's a great book.