What is art?

This is a question that has been going on since time began and will do until it ends. But it is still important to consider. Certainly art is subjective: each person has their own view on the matter.

Leaving aside the sort of stuff that gets featured in the turner prize each year, the most controversial point is sex in art.

Nudes have been the gold standard of western art for nearly 2000 years. I recently read an article in which a conservative Christian college in the states defended and explained their practice of including nude life drawing classes in their senior art program. So it clearly seems that the human body isn't that big a deal on it's own.

So what is? Is it the manner in which it is depicted? The motivation behind the depiction? The viewers own personal standards or reactions? The medium that the piece is done in? The reaction the artist intended to provoke?

Thoughts please.