differences between the two words?

Welcome to A2K, ww. I believe that a prelude is more like an introduction to a book or movie, while a prequel goes back to the very beginning, as in a movie, etc.

A 'prelude' (which has more to do with music than literature) would be an introductory story leading into the main narrative of a book.

A 'prequel' (recent coinage) is a story whose narrative precedes (in fictional time) a continuation of the story which was actually written earlier; some of the Star Wars movies are cinematic prequels.

to Mister Micawber:
I don't quite understand the word "continuation" in your sentence.Can I put it like this?When I say a prequel of a novel,does it means it is a story written based on published novel but the events in the story taking place earlier?
New spring is a prequel to the Wheel of time,right?

Hi WW,

Sorry, I don't know 'New Spring' or 'Wheel of Time'-- and I can't even tell you which Star Wars movies are prequels to which-- but, yes, 'a story written based on one's previously published novel but the events in the story taking place earlier' is correct.

Thanks for your help.