Defining your book's genre gives it boundary for both the writing and the marketing. Here is a comprehensive list:
Defining the genre for the BryonySeries has been difficult as it does not neatly fit into any couple of categories. It has elements of literary, gothic, fantasy, psychological suspense, historical romance, and magical realism. I've also pegged it and intended it as young adult, although I have several very happy adult readers (and it's main audience has been adult) who would argue with that classification.
I'm thinking I might have to invent my own genre for the series.