A book or story connected to a previous one through using the same characters or continuing the previous plot. The original plot is usually brought to a conclusion and the new story creates a new plot for the same main characters, or picks up the lives of lesser characters from the first story. The sequel is usually written by the original author, but it can be written by another author if that person gets written permission to do so. Sometimes a publisher will hire a number of different writers to each write one of the sequels. In any case, getting that permission is required, since the characters in the original story belong to the original author. Even if the original author is deceased, those rights belong to his/her heirs and permission must be sought from them—unless the original book has gone into public domain. In that case you would not need permission, but should ethically credit the original author in the acknowledgments. Here are two sites that will help you understand and write a sequel: http://blog.janicehardy.com/2011/03/going-once-going-twice-writing-sequels.html, and http://www.davidlouisedelman.com/film/writing-sequels.