The personal history of each of your characters in fiction, as well as the world in which it happened. Even if you created the characters yesterday, they still must have experiences in their background that contribute to who they have become. A well-crafted back story will help bring your character to life on the pages of your book. Often it is the back story from which the plot—or some elements of your plot—will emerge. Although you may create a full history for each of your main characters, few of those details will actually be revealed in your story. As the author, you will simply know how your characters will act or react based on their personal history. For a closer look at back story, go to: http://theeditorsblog.net/2012/03/30/when-to-bring-backstory-out-of-the-shadows.