What is fashion?

Definition:
1. a prevailing custom or style of dress, etiquette, socializing, etc- the latest fashion in dresses
2. conventional usage in dress, manner, etc- especially of polite society, or confomity to it- the dictates of fashion-to be out of fashion.
3. manner, way, mode- in a warlike fashion.
4. the make or form of anything- he liked the fashion of the simple
http://dictionary.reference.com/browse/fashion
 
The word fashion is very loose term it means a lot of things, but what i am trying to find is what fashion as a whole has done to people, what is behind the clothes, beauty, advertising, and life of fashion.
who determines whether you are too thin or fat?
who determines what is in and what is not?
why is fashion so important to us in everyday life?

This projoect is going to look into both fashion and ballet and look at the horrible side or both businesses.