A fangirl is the worst kind of female fan: the kind that obsesses over a character or work ad nauseam while completely missing the point of what that character or work is all about. She will wallpaper her room with posters of her lust object, but will fail to capture his character in any way in her fanfiction. She will pepper chatrooms and author's notes with praise for a canon character's "hottness" (two "t"s included) and will generally demonstrate zero maturity and emotional restraint. She hates her lust object's significant other, couldn't care less about canon, and populates the biggest fandoms to a terrifying degree. Harry Potter, Lord of the Rings, and Pirates of the Caribbean are all infested with fangirls, who are consistent perpetrators of Mary Sues and canon destruction.
The overwhelming majority of writers in these fandoms are female, so the fangirl's counterpart, the fanboy, is rarely encountered. When he is, the Canon Protection Initiative treats his work with the same contempt as that of the fangirl.
There is a cure for the common fangirl. It is known as the Official Fanfiction University. OFUs advocate forcing fictional fangirls to learn, through pain, to be better writers. The OFU fangirls will attempt to glomp or stampede on occasion, but are always held back by their wardens, the university minis. Fictional fangirls are also the chosen cuisine of mini-Balrogs.
Fangirls and the PPC
Several PPC agents are OFU graduates and thus ex-fangirls. Such agents are encouraged to attend Lusters Anonymous.