Fan fiction is when a story is written about a celebrity, band, TV show or movie, using the characters that are already well-known or previously created. This is usually without the creator's permission but is protected under the U.S. Constitution and the First Amendment.
Fanficporn is then taking these characters and introducing them to porn-related plots and situations. If it is a need to flesh out a background, or an interaction that was handled away from the viewer, fanficporn can serve a purpose other than of an erotic nature. Or it can.
Celebrity porn is a whole section of fanficporn dealing with such well-known series as: Buffy the Vampire Slayer, Star Wars and comic book entities. You will be amazed on how they rationale the characters to have sex.