Popular Fiction is also known as Genre Fiction. While some may not truly understand the exact nature of Popular Fiction, the defining of it is quite simple.
Popular Fiction is defined by a work of fiction which is easily identified by the theme of the text. What this means is that a reader familiar with the material presented in the work can easily define it as part of a collection. Popular Fiction is not a work which is new to the "shelve" or novel in its ideas, characters, or theme.
Essentially, Popular Fiction encompasses a very large number of subcategories, or genres. Romance novels are considered Popular Fiction, given all romance novels are, essentially, the same. Therefore, any text which is easily identifiable by readers of one specific genre can identify the text as Popular Fiction.
That said, one can identify a text by its similarities to others. For the most part, the sub-genres of Popular Fiction are limitless. Examples of these sub-genres are Horror, Suspense, Mystery, Crime, Sci-Fi (or Science Fiction), and Westerns.
Popular Fiction is simply a title given to the majority of all short stories, novellas, and novels. Outside of being defined as Popular Fiction, the work also fits into another category, which aligns with the material presented in the text itself.