The art of obsessing over fandoms, the characters in the fandoms, the actors who play the character ( if the fandom is a film/show ) etc.......
Just saying something to do with a fangirls fandom will usually lead to them frantically crashing into the room youre in making high pitched sounds that to you might sound meaningless, but to other fangirls will make total sense

v. 1. the reaction a fangirl has to any mention or sighting of the object of her "affection". These reactions include shortness of breath, fainting, highpitched noises, shaking, fierce head shaking as if in the midst of a seizure, wet panties, endless blog posts, etc.

2. a gathering of two or more fangirls in which they proceed to waste endless amounts of time ogling, discussing/arguing, stalking, etc. the object of their "affection"

See also:
fangirl, glomp, squee, groupie

Any girl extremely obsessed with any celebrity, t.v. show, movie, book, or other entertainment/media who is participating in fan actitivities such as message boards, fanfiction, or conventions.

Has been used in the past as derogatory but has become less and less in certain fandoms. Fan girling frequently happens with large amounts of squee.

The noun would be fangirl or occasionally fengirl, though the later is usually used as plural.

From the noun fangirl

V. To make loud high pitched noises (this includes squealing, yelling, or talking so quickly that it doesn’t sound like they are actual speaking a real language) or show their excitement for the object of their obsession though movement (this includes jumping glomping, running in circles, etc.). Fangirling involves a lot of physical action unlike fanboying and can be quite annoying.

One does not have to be a girl to start fangirling though most girls do.

Also see fanboy

The action of which a fangirl is freaking out over something related to their fandom. Usually it involves the following: Screaming, squealing, crying, other weird noises, threatening to kill someone, and violent actions.

The act of when a girl who is extremely passionate and crazy about a certain book or movie squeals/screams hearing about the latest info.