Harry potter and Naruto are some examples of fanfiction. From the word itself, these are fictions made by fans, it did not actually originate from the creator. The creator will get ideas from these works. There will be of course modifications and improvements on some parts.
Fanfiction is a popular form of fan labor. It is rarely commissioned or authorized by the original works, creator or publisher and is rarely professionally published. They come in some categories such as specialized fanfictions and traditional fanfictions. Traditional fanfictions include genres such as horror, fantasy and romance. On the other hand, specialized one focus on things like slash or shipping.
Depending on the jurisdiction, it may or may not infringe on the original authors copyright. Not all fanfictions are qualified to be published and authorized. Attitudes of authors and copyright owners or original works to fan fiction have ranged from indifference to encouragement to rejection. Fan fiction is being both related to its subjects canonical fictional universe often referred to as canon.
Most fan fiction writers assume that their works are being read by other fans and thus presume that their readers have knowledge of the canon universe created by a professional writer in which their works are based.
A good fanfiction has a good idea and concept. If you are planning to make one, come up with an idea, formulate to a story and proceed to writing. Consider the impact to readers and how will it entertain the public. You can be a writer!