A fangirl* is an individual, regardless of gender, who is mildly obsessed with a particular person(s). While a fan girl can be a fan girl for multiple peoples they likely have one person or group of persons that they fangirl the most over. The act of fangirling is a ceremonious, almost sacred event. Most people consider fan girls to be silly individuals who obsess over people they have no chances of ever being with, but little do they know that fangirling is much more serious.
One subset of fangirling is fangirling over one individual. This can involve buying all of their available merchandise including clothing, music, fragrances and posters. One common group of fangirls are Beliebers who fangirl over Justin Bieber. They have been known to wait for hours before his concerts just to get a glimpse of him. These fangirls can be very nice by supporting their idol and sending them pleasant gifts but occaisionally the line gets crossed and they get nasty. For example previously while Justin was doing drugs some Beliebers started a self harm movement which was detrimental to the population. Though much of it was a hoax some Beliebers were indeed affected and this is where fangirling can go too far. Fangirling can also expand to groups such as One Direction and various KPOP bands.
Another subset of fangirling are the shippers. Shipping is the act of pairing up people into couples whether they are in one or not. Shipping can occur in TV shows, comics, cartoons, and even amongst celebrities. One show that’s characters are commonly shipped is Avatar. The largest ship by far is Kataang but there is also Zutara. These ships can be canon, when they come true in the show, or they can simply exist in the hearts of many like Zutara. The two sides of fangirls who ship these couples often feud over who is right or wrong but in the end often reach mutual agreements over the greatness of a show in general. A new tv show, Sleepy Hollow, already has a major ship. This involves a man named Ichabod and a woman named Abbie. When the fangirls got a hold of them they squeeled with joy. There is already fan art of the couple and various posts on tumblr about how the fangirls need them to be together.
The broadest (but not the last) subset of fan girls are those that belong to fandoms. A fandom is essentially a kingdom of fans. These fandoms usually refer to TV shows, but also books, movies, video games, and other forms of entertainment. The fandoms can be found battling over whose is best but they can also be found supporting each other. A friend of mine mentioned to me once how when the Homestuck fandom was sad because Homestuck was on hiatus, the Hetalia fandom supported them and they all became friends. On the other hand, the Dr. Who fandom (Whovians) and the Sherlock fandom often butt heads about whose show is better even though many members avidly watch both shows.
Regardless of your subject of choice, you can be a fangirl for anything and anyone be yourself and fangirl as hard as you possibly can. If you ever need a place to exert your fangirl feels be sure to check out the ever so beautiful land of tumblr and browse the tags for friends.
*While fangirling can be extreme, it is part of a wide spectrum that contains varying degrees of severity, and not all fangirls can fit under one umbrella term.