This is a question, that I've asked myself many times over the years. Normally, when it came to movies, I never really worried too much about it. Now that I've chosen to go down that route with my writing, it is something I ended up wanting clarification on. Not to mention, with it being October, it seems appropriate to talk about it.
What I've found out is that it can be very much dependent on the reader and subject matter. No two persons ideas on these two genres match. With this being October, I figured, I would go ahead and put my spin on things. (This is my blog so why the heck not)
Horror: For most people, this is typical in a movie/novel which involves horrific events. People are dying and the most horrible ways and we see most, if not, all of it. We see/read the deaths, how they die, the torture etc. Most times, this is done by the introduction of an evil force or entity. This can be from something supernatural or a character that is psychotic. It used to be that many of the types of characters you would see within the horror world would involve creatures such as zombies, vampires, werewolves etc. The line on that has blurred as writing and movies have evolved.
For example: Horror: SAW (series), Night of the Living Dead, Resident Evil, Psycho
With the above, those would definitely, in my opinion, fall under the realm of straight horror. SAW you have a psychopathic killer who takes pleasure in torturing and killing his captives and we see every part of it. Night of the Living Dead and Resident Evil series we are tossed into a world where there are undead zombies walking around eating people's brains. Again, straight horror for that one. Psycho? Again, a human killer who is mentally unstable.
Those examples are classic horror. They are pretty easy to identify and hard to miss. Now on to Thriller.
Thriller: This area is a bit broader and can overlap with many other genres (including horror). With this genre, you are looking more for the sense of suspense and uncertainty. This can be from many character points of view, but the underlying factor is, suspense of not knowing what is coming or going on. It is to keep you on the edge of your seat guessing. This can, at times, be much more difficult to do than horror. Most of what is going on isn't obvious or able to be seen by the naked eye because it is based on sensations and feelings.
Examples: Twilight Zone, Buried, American Haunting
The three above are just some examples of Thriller. None of them really show anything horrific, but do keep you wondering. Hitchcock was a master at environmental influences and setting the mood. Buried, was another one that made you wonder what was going to happen and when you thought you knew, you found out you were wrong. American Haunting was also a great example of tripping the watcher up at the last few scenes. In most Thrillers, there will be some kind of sense of forboding, like shadows or evil just out of reach. That evil can be very real or imagined. In the end it is the setting and what you can't see that can distinguish it as a thriller or horror.
Now with that said, there are, and always will be, a grey line between the two genres, with one going into the other. It is what makes creativeness so much fun. A writer is not forced to remain within one genre, but can free float between them. It is what allows us to make our worlds, events and ideas our own.
In the end, how you view these two genres is up to your tastes and opinions. Personally, I think that Horror will always have a snip-it of Thriller in it whereas, Thriller may not always have horrific elements involved.
With that said, I hope you've enjoyed this as much as I have. And soon, I'm hoping to have my Horror novel out soon.
Happy writing or reading or both!!