A girl with an obsession of something. Can be a celebrity, anime, a band e.t.c. popular obsessions are Orlando Bloom, Elijah Wood, Inuyashamashywhaever, and Good Charlotte. Most of them do have lives outside of their crush apart from the odd scary one. They can produce quite good fanart and fanfiction. Many are good-looking and thin (e.g. many of my friends). Many are also nice people who are capable of answering a question with 'squee' or 'OHMYSEXGAWD'. I am a fangirl and proud.
Crazy fangirl: OMG it's him!!111!!oneone I luff you marry me!!!11!!!
Normal Fangirl: Wow look it's *whoever* he looks good. Can I have your autograph?
Orlando Bloom's nickname which in fact is not just used by fans. His friends call him it too e.g. Elijah Wood and Sean Astin. (I discovered this while watching the Extended Edition of FOTR with the cast commentary on.