A spoof is a funny version of something, like a movie or a book. The classic comedy "Young Frankenstein" is a spoof of earlier, serious film adaptations of the famous novel.
You can also call a spoof a parody. A spoof borrows material from the original, copying the style and characteristics that make it obvious what the subject of the spoof is. Movies like "Spaceballs," a spoof of the "Star Wars" films, and "Scary Movie," which spoofs the entire horror film genre, are great examples. Spoof originally meant "hoax," and it comes from a game, Spouf, invented by a British comedian in 1884.