Fanfiction

What is a Fanfiction?
A Fanfiction is a story (or collection of stories) written by a fan (or many) about their favorite fandom. A fanfiction could be a quick one-shot or a full-blown story. Popular fandoms for fanfictions are:

Harry Potter

Wings of Fire

Warrior Cats

Fanfictions are sometimes written as the reader being Y/N. Others are written as the reader following the main character.

What's the diffrence between an AU and fanfiction?
An AU (alternate universe) is a genre of fanfiction that uses characters from the actual book seires or video game but puts the characters in a diffrent situation or time period. While an exsample of a fanfiction that isn't an AU could be (using Warrior Cats) diffrent clans, leaders and diffrent setting.

To be honest, the diffrence between a AU and fanfiction is just the style of writting and where the characters are.