Warning: Wikipedia contains spoilers
The Princess Bride is a book by William Goldman. The premise of the book is that it is not Goldman's original work, but an abridgement of an older version by H.R. Morgenstern, which was originally a satire of the excesses of European royalty. Goldman, however, remembered the book from being told it by his grandfather as an exciting adventure tale, without the complex political overtones. His work is a recreation of the abridgement of his grandfather. In fact, the book is entirely Goldman's original work, and Morgenstern and his "original version" never existed.
The movie, The Princess Bride adapted by Goldman from the book, is in essence a fairy tale, but it's not just a children's movie. Directed in 1987 by Rob Reiner, The Princess Bride stars Robin Wright (who has since married Sean Penn and appended his last name onto hers) as the title character, named Buttercup. The beautiful Buttercup falls in love with her lowly stable boy Westley, played by Cary Elwes. Westley leaves, promising to return, but his ship is attacked at sea by the Dread Pirate Roberts, who is notorious for taking no prisoners. Fearing him dead, Buttercup eventually agrees to marry Prince Humperdinck (Chris Sarandon).
Buttercup is kidnapped by a bizarre trio of outlaws, Vizzini (Wallace Shawn), Inigo Montoya (the superb Mandy Patinkin), and Fezzik (the late Andre the Giant). Westley returns just in time for to rescue Buttercup, and must defeat all three of her captors in order to do so. The two escape the kidnappers, only to be captured by Prince Humperdinck and the menacing Count Rugen, played by Christopher Guest. Buttercup is returned to the palace to await her wedding -- which, now that Westley is back, she no longer desires -- and Westley is taken by Count Rugen to the Pit of Despair, where he is tended to by an albino (Mel Smith).
In the ensuing action, Westley again encounters Inigo Montoya and Fezzik, as well as Miracle Max, a washed up wizard, and his wife Valerie, who are played by Billy Crystal and Carol Kane. As you might suspect, the good guys win, and the bad guys get their comeuppance.