A prequel (a portmanteau of 'pre' + 'sequel') is a story that precedes a previous story in chronological sequence. Prizzi's Family, a satirical novel by Richard Condon, for instance, was published in 1986, four years after the acclaimed Prizzi's Honor, but its events take place about a decade earlier. Prequels do not usually achieve the fame of the original stories on which they were based, but they do occasionally become recognised in their own right. An example of this is Wide Sargasso Sea, by Jean Rhys, a prequel to the much better known Jane Eyre by Charlotte Bronte.
A story that follows a previous story in sequence is known as a sequel, and is a more frequent literary device.