In the comments for Did any actor ever win an Oscar for their work in a horror movie?, there was a lot of discussion over whether or not a thriller should be considered to be a horror movie.
IMDb classifies Rosemary's Baby to be a Drama/Horror movie. However, Black Swan is classified as a Drama/Thriller movie (and at least one pundit has said that it was really a horror film that only got an Oscar because it was able to not be associated with the frequently award-snubbed horror genre). Then there are some movies like Blair Witch that are both Horror and Thriller.
What exactly is the difference between these two genres? What would cause a movie to be a horror movie, but not a thriller (and vice versa), and what would cause them to be both?
(I imagine that there are film scholars and such who have weighed in on this, and I'd like to see something more authoritative like that instead of just the average Joe's opinions).