Harry Potter materials that are considered to be fan fiction are anything that has been written in the Harry Potter universe that was not written by JK Rowling herself. Anything that is written by fans and not by the author is considered fan fiction.
Justin Bieber hasn't said anything about Dave Matthews Band. I am a fan of Dave matthews and a big fan of Justin Bieber. Based on previous answers when people asked him what was his favorite band, It seems like DMB is something that Justin Bieber would like
The stories that are featured on the Harry Potter Fan Fiction website are written by fans of the books or films of Harry Potter. They are written as if the author is in the Harry Potter story. Some of those stories are Never Alone, Days of War and Stolen Freedom.