Who casts a film?

I thought that I would do some research on who actually casts a film as I always thought that it was mainly the director's job to do so, but from my findings its not.
The casting director is the main person who casts a film, from choosing the main role down to extras used in the background. They are given a rough image that the director imagines to play the part, for example he may give them the gender and the persons particular age/hair colour/skin colour etc, and the casting director has to find someone who fits the bill.
When casting an actor an audition process is generally taken out and the casting director is given the final word about who plays the part.
Famous casting directors include Kerry Barden who casted films like American Pyscho and Se7en and Sheila Jaffe who casted Rambo and The Italian Job. This illustrates that they each have a particular genre that they mainly specialise in to make them appeal to production companies looking for casting directors.