The funny thing about fashion theory is that everyone has their own take on what it is, what it means to study and research it, and what words to use to describe it. As a matter of fact, even the words used so often in the field; fashion, style, dress, costume, are still debated in terms of what they are referencing.
Maybe the easiest way to talk about fashion theory is by looking at what it is not; it is not a history of clothing, it is not trend forecasting, it is not celebrity fashion, it is not haute-couture catwalks or rodeo drive, it is not fabric analysis, it is not fashion advertising, it is not any one of these things in particular, yet it is the common thread of study of, well, all these things and more in a way where we can deduce meaning about a society, culture, nation, or any group of people.
Fashion theory scholars, apply concepts from a variety of disciplines; cultural studies, women and gender studies, anthropology, queer theory, feminist theory, sociology, social psychology, in order to take the seemingly monotonous act of dressing oneself everyday and apply it to study everything from the micro levels of identity and self concept to the macro level of power relations, and the hierarchy of social systems.