What is a fake girl?

I am trying to get thru high school being as real as I can. Can anyone tell me their definition of a fake girl?

im not saying all but most popular girls are fake well, thats what they where like when I was in school, there really ncie to your face an then stab you in the back!!

fake is trying to be something that really isnt you or doing something you wouldnt do.

I found a couple of different things on the internet that explain it all...
'A person whom I think is fake can be very easily detected. For example a person who will act like a best friend to someone then the minute they turn their back the person is saying how much they hate them etc
Also you will notice they put on their laughs, smiles and try to please everyone ( to their faces). You will know it when you see it trust me.'

'it means that they are pretending to be some one there not .the signs are laughing in a way were it's forced ,it's really when they do things that aren't like them'