When is it Horror and when does it become something else?

You'll find a lot of different definitions of horror, but the one I'm fond of is if the goal of your work is to frighten the readers, then it's horror. Obviously it's not as black-and-white as that, though, so you may have to wait for some additional opinions.

Post-apocalyptic action-adventure, maybe?

When I try to determine if something is horror, I think back to a series of dreams I had as a teenager. If I detailed them here they would sound like nightmares. I frequently fled or fought for my life. Despite the subject matter, however, I wasn't frightened. I thought of them as adventure dreams.

If you fell asleep and were plunked down in the middle of your story, would it be a nightmare that would wake you up in a cold sweat, a scream caught in your throat? Or would you wake up and be like, "Wow, that was nuts! I think I'll doze off again and see if I can find out the ending."

Like Niccola said, if it's scary it's horror. There is no reason horror cannot have inner challenges, family, fun, sword fighting, etc. too. What makes horror horror is that it frightens people.

Horror/Satire. ?

In my experience, horror is defined by sub-genres. I participated in a similar discussion once regarding whether or not The Monster Squad and other horror-comedies belong in the same place of discussion as intensely terrifying films. Generally, any film fitting a horror sub-genre is accepted by horror fans because they have those elements.

I wouldn't market a horror comedy to an agent that exclusively works with serious horror, but a horror-drama? In the modern day, that's what most pieces are. More and more, the entertainment industry is taking into account the fact that relatable characters with human challenges are the key part that will hold their readers that were hooked by the horror aspects - hell, look at The Walking Dead, for instance: a horror show in which the audience primarily discusses the human aspects.

Horror is intended to scare people. There's so much out there that's classified as horror, but was never really meant to be. And there's this idea that zombies/other monsters automatically means horror, which it doesn't. Not sure where that came from. Maybe it's because said monsters were originally used in horror only, and people can't get that idea out of their head. If you have a zombie apocalypse story, it might get automatically classified as horror just because that's what genre people would want to put it in...but it may or may not actually be horror.

If horror is supposed to scare me, I've never read a horror story. Do real people actually get scared over what's written in a work of fiction? I know zombies, vampires, etc., are not real, and they simply do not frighten me in the least. If I want a novel to frighten me in any way, it needs to be a story I not only believe can happen, but that stands a very real chance of happening in the near future.

I read horror novels for the same reason I read westerns, mysteries, science fiction, or the Harry Potter novels. I read because I care what happens to the characters. I expect the characters to be afraid because they're part of the story, and to them, the monsters are real.

But I also expect the characters to be frightened if we're invaded by aliens, or if they learn Russia has just launched every nuclear missile they have, etc.

I have a very hard time believing that many grownups are actually frightened by what they read in a horror novel. Kids, sure, but grownups? None I know. If it were phrased, "Frightened for the characters", I'd buy into it, but that's it.

It's horror if it uses the tropes of horror. Go to a horror convention, and you see fans who take in almost anything as horror, and it's just silly to say they're wrong.

But scared by a horror story. No, not hardly. I can be disgusted, and I can be creeped out, but almost never in a good way. I sure as heck can't be scared by a horror novel, anymore than I can be scared by a western, a mystery, or a historical romance. If I judged horror by what scares me, there is no such thing as a horror story.

Fiction intended to scare/unnerve/disgust is a good, broad definition, but there is no hard and fast line that separates Horror from other genres. A lot of people are willing to take anything that mentions a subject they associate with horror or scary things and they think that makes a book horror.

I recent read Dark Places by Gillian Flynn. A lot of people call it a horror novel, (it was voted for a group read in a horror group.) but I think it was actually a mystery-thriller. The only thing that might pull it into horror is a suggested sub-plot of satanic worship, but it really has no bearing on the story.

Really, there are a lot of broad definitions of what horror is, which you can write under and be safely covered, but it can easily be written up to the opinion of the reader too.

Poe

My favorite type of horror is psychological horror. I just got done done reading the tell tale heart by Edgar Allen Poe for the first time since 20 years ago. I like how he hate the man with an evil eye and then kills him then the guilt in the murderers mind forces home to admit the truth there is a psychological horror in his mind that is paranoi, and I think it is my favorite kind of horror.