The Princess Bride can refer to:
I've seen the film but not read the novel (shame on me), so I'm curious as to what differences there are in the plot, the characters, etc. between the two versions of the story.
There's quite a bit, but let me sum up...
In the movie, the story is being read by a grandfather to his sick grandson, and that's all there is to it.
In the novel, the setup is close but not quite. The author states (lies) that the novel is an abridgement of a much larger satirical renaissance work subtitled "S. Morgenstern's Classic Tale of True Love and High Adventure". It was (still lies) the book that his father read to him when he was a child, but skipped bits he considered to highbrow or boring for a kid. His abridgement is in theory is an attempt to recreate that experience out of the original work, with running commentary on what gets skipped and musings on his experiences. This means the novel reads a lot like the movie - as in, with many interruptions - but the interruptions are usually more adult-minded than "Is this a kissing book?".
In the movie, the Pit of Despair is just a hidden room underneath a grove of trees. It's a well-hidden torture chamber, but that's it.
In the book, the Pit of Despair is the bottom room of a multi-level dungeon named the Zoo of Death, containing all of the many many lethal creatures Humperdinck has demonstrated his strength against. Inigo and Fezzik have quite a bit of work to do to get to Westley. There is a good bit where a deadly trap of doooom awaits our heroes at the bottom (a deadly spider hiding in the doorknob), but Fezzik either being genre-savvy or fourth-wall-aware gets scared and just beats the door down.
In the movie, the opening moves swiftly along and despite the intervening years Buttercup is engaged to Humperdinck mere minutes after Westley is murdered by pirates.
In the book, Humperdinck has small bits of plot to set up the later hijinks, where he rejects Guilder's princess, approaches Buttercup to propose (which she treats with disdain, but having few choices accepts), and everything's more fleshed out by the time we get to high adventure. Humperdinck himself is portrayed as a mighty hunter and a "man's man" without peer, but this is tied into his ultimate flaw, cowardice, and shown to be vain attempts to demonstrate his dominance against things he knows he can beat. Most of the bits that Goldman fake-skips are early on as well, so there's more interruptions as he explains that forty pages of Florin history and sixty on the attempts to restore the old King's health are useless bunk, as is Buttercup's time being princess trained and Humperdinck wrangling the nobles into accepting the marriage.
Much of the rest of the cast gets their buildup too (as @FuzzyBoots notes in comments). Rugen's reasons for seeking Inigo's father out are clearer; his having six fingers isn't just an identifier, it's a handicap to the balance of a master swordsman, thus the point of his special commission. Fezzik's childhood as an utterly gentle soul is gone into, and that turns out how you'd expect.
The finale matches the movie quite a bit, but I'd be remiss if I didn't mention that the author finds out that his father skips the final section of the tale, where while riding off into the sunset, Westley relapses and falls off his horse, Inigo's wounds reopen, Fezzik tries to lead and takes a wrong turn, Buttercups horse throws a shoe, Humperdinck and his hunting party can be heard catching up, and so the adventure continues on a very bleak ending note.
There are even notes in some editions of the (still fake) sequel titled "Buttercup's Baby" that the author is fighting with Morgenstern's estate to get the rights to.
To finish up, a bunch of little things...
Finally finally, I should also note that lots of the dialogue is lifted straight from the novel, although some of it is in slightly different order. Almost every classic and quotable film line is in there (except Miracle Max, but even that's closer than you'd expect to Billy Crystal's improv), and I had started a list of similarities before it grew too large and silly to address.
The movie is inside out from the book.
The movie is primarily about a fantasy story book called The Princess Bride being read by a grandfather to his sick grandson. The storytelling is just a frame for the fantasy story.
The book also features a fantasy story book called The Princess Bride being read to a child, but the book is about the relationship between a father and son, and that how you tell a story is the important part.
The book focuses on the author trying to connect with his estranged and distant son by telling him the story of The Princess Bride. The author has fond memories of being told the story by his grandfather. He gives his son the book, hoping to build a similar connection, but is heartbroken and confused when his son doesn't like it. This relationship between father and son is the focus of the book, inter-spaced with the story of The Princess Bride to frame it.
It turns out the author never read the book, he was read it by his grandfather. Upon reading the book himself he discovers it contains long winded, overly detailed, genuinely boring political passages. He realizes his grandfather was editing the story, giving him "the 'good parts' version" that a kid would like. He learns the important connection wasn't the book, but how his grandfather told it, and the time they spent together.
The book is not a novelization of the movie. And the movie is not merely an adaptation of the book. They are both excellent, but they are both very different. Like the book presents "the 'good parts' version" of The Princess Bride, the movie is the "the 'good parts' version" of the book.
If you just want to read the book to see the differences in the story of The Princess Bride... I'd skip it. Stick with the movie. If you want to read an excellent story about the relationship between a father and son punctuated by a great, tongue-in-cheek fantasy story, read the book. It's a great book.
