Typically a teenage girl with an obsession towards something: a show, movie, book, person, etc. There are three types of these fan girls:
Normal fan girl:
Perfectly normal girl, may even be pretty. Has an obsession, but keeps it to herself. Might read fanfiction, have a blog dedicated, several merchandise related to this obsession. Safe to approach.
Crazy fan girl:
Relatively normal girl, sometimes pretty. Really enjoys her obsession. May talk about it every once and a while. Reads fanfiction daily, and writes it. Dedicates 1-2 hours of her life to this obsession. Owns several pieces of merchandise. Cosplays every once and a while. Approach with caution.
Completely insane fan girl:
Needs to go to a mental facility, usually does not care about beauty in any way. Rarely pretty. Loves her obsession (typically anime) more than anything in the world. If you took away her internet access, she would retreat to her army of merchandise related to her obsession. But, after a few hours, would die of lack of fanfiction. Cosplays almost every weekend. Dedicates 6-7 hours of her life to her obsession. Spends the rest of it thinking/dreaming about it. If you encounter this type of fan girl, call the police immediately. DO NOT APPROACH.
Normal fan girl: "My favorite show is NCIS. I really like it. What's yours?"
Crazy fan girl: "OMG I love Harry Potter so much. My favorite character is Draco. I know he's a bad boy, but I can't help but love him. *SQUEE*"
Completely insane fan girl: "Hetalia is like the greatest thing ever. Look, I've got my Italia shirt on. I'll wear my bracelet and headband tomorrow, and I'll even bring in my stuffed Chibitalia collection to show you! It's the greatest show in the world. I stay up every night and just watch the episodes over and over and over again. Read all my fanfics. I've written over 2000! OMG *SQUEEEEE* Sorry, I just got some major feels. I ship so many different characters and, and *SQUEEEEEE* just thinking about them makes me want to write more fanfics. Let's be BFFs and talk about Hetalia all day long ^_^"