Fanfiction (shortened to fanfic or fic) is the art of writing a fictional story using pre-existing characters and worlds, whether they're from TV shows, movies, books, video games, or even Internet Flash cartoons.. Original characters can be introduced (e.g. a new student at Hogwarts) as well. Some fanfics can be crossovers between two different series or worlds (e.g. Harry Potter meets Inheritance or Grand Theft Auto meets Super Mario Brothers).
There are good fanfics and fanfic authors out there, however, fanfics tend to be associated with many negative elements, such as; Mary Sues, bad grammar and spelling, cliche plots, archetypes, purple prose, bad writing style, and absolute butchery of the actual characters.