Fangirling means going totally insane over a famous person (/over famous people, like a band,..)
Usually happens when you find out something about your idol, which makes you freak out.
Fangirls also have a special language, which means that they can't type when they're fangirling.

When they're fangirling they like to use the word 'sunshine' ; that's how they call their fav famous person.

A Girl who worships certain bands (often untalented bands), celebrities/characters (douchebags such as Edward Cullen and the Jonas Brothers), TV Series and so forth. They're often extremely annoying and disrespectful of the opinions of others, especially if they oppose their own. They can be easily confused with Teenyboppers given their many similarities such as using low-quality cell phones often plastered with countless charms, stickers and such. Often times, they will persistently behave immaturely and never hesitate to promote whatever it is they love. I could go on about this for pages and pages but to put it shortly...Fangirls are fucking annoying!!

when screaming fangirls attack celebrities by way of hugging and not letting go, kissing, screaming, etc. Usually these are teenage girls

to squeal or behave like a teenager, no matter your actual age, in response to anything Twilight/Edward/Rob Pattinson related. May also cause shaking, crying, shortness of breath, spontaneous Robgasm, or Jizzing your pants.

A girl usually about 12 that will obsess over someone that does not care about her, and never will. Fangirls are usually unable to see that the person they are obsessing over has no skills or talents and actually isn't as famous or as good as they think they are. Fangirls like their obsession because of their looks more than anything.

A crazy person (if you would like to call them people) who obsesses over a fake or real person who is in a movie and/or on television. These creatures usually spell horribly. Most of them devote about 75% of their time to the Movies & Actors Board at YM.com, only talking about how much they love their victim. Their usual victim is Orlando Bloom, also known as Orlihobbit. But they have many more victims that they can not stop talking about.
I recommend using one of my special black ink guns to knock them down. (and get them very messy) A spork or a hose are wonderful substitutes.

A fangirl is a girl who found something, like a book or a tv show, and liked it.
They have a roller-coaster of emotions, and will burst at any second. If you mention something she likes, or someone she obsesses over, she will literally explode with fangirl emotions.

How do fangirls fangirl, you might ask? Well, let me educate you, child.

How it normally goes is they gasp, then jump up and down, and make many strange noises, typically squeaks and shrieks. Then, they will proceed to speak about said thing at the speed of light, listing off every good trait about that thing. Sometimes they get so hyper they cry, so watch out.

Also, don't anger a fangirl. If you have an opinion about something, and your opinion is negative towards her favorite band, show, book ect, I would suggest you run. I mentioned once or twice they have many emotions, so trust me when I say they're full of anger too. You could literally shatter a relationship if you said something offensive towards the things she's obsessive towards. I have literally kicked people, bit them, and jumped on their backs and hit them, because they said something I didn't like. So like I said, watch out.

Fangirls have pretty high standards when it comes to men. I mean, they fangirl over people like Gerard Way, Fred Weasley, Sherlock Holmes, Daniel Howell ect. But of course she has a senpai, or crush, if you don't know senpai.