I know the classic definition made popular by B-movies, but I was wondering what people thought the present definition(s) of the genre is.

Someone mentioned the Twilight books and such, but those seem to be shelved in YA and, honestly, generally discussed as YA.
What he said.Twilight IS NOT HORROR!
Twilight is a thinly disguised romance with hints of "the bad boy" disguised as a vampire story. IT IS NOT HORROR!!!
Stephen King's Under The Dome is not horror (it's psychological suspense). Stephen King's Pet Sematary, From A Buick 8, and Cell are horror.
Brian Keene's The Rising and The Conqueror Worms are horror.
Dean Koontz's The Taking and Midnight are horror. His books Watchers, Lightning, and Dark Rivers of the Heart are not.
People think they're reading horror when what they're actually reading is psychological suspense or a thinly-disguised piece of romance in many, many cases (most these days it seems). Readers like what they like and I'm not faulting either them or the authors who feed them for that. I AM absolutely faulting the publishers who publish an author's work as horror when it clearly is not and the publisher clearly does not have Clue #1 as to what horror really is. They pollute the genre, decrease the genre's value as a genre in and of itself, and lower the effort it takes to create a true horror story in order to push a book out into a genre that it ordinarily would be laughed out of.
Horror is a state of mind and a genre of writing that brings to mind certain states of mind and emotional experience.
That's probably the best clinical description or definition I can come with on the fly.
But more than that, horror is that deep, buried portion of our reptile brain that still fears the things that go bump in the night. It's the breath of wind that stirs through the leaves and the grass that leaves us wondering what might be out there - or worse what might already be in herewith us.
It's lying awake at night listening for the baby's next breath and fearing it might not come. It's doing something we're ashamed of and wondering when someone's going to find us out.
It's seeing the red and blue lights come on behind us when we've been out too late and had a couple too many. It's watching a parent or loved one with a horrible disease descend into pain-ridden madness and rage at the doctors and nurses and disown their children because they can't or won't end the suffering.
It's waiting on the flight line knowing you're carrying the bomb that could end the war, but will kill tens of thousands in a single blast.
It's looking at the kitchen garbage disposal and wondering what it would feel like to stick your hand down there when it's switched on.
Horror is a thousand little things that we see and do every day which we know can suddenly turn around and bite us.
IA, we agree to disagree.
Ultimately, as someone way above my lay grade put it, horror is ultimately romance.
However, Tommyknockers is horror because a)there is a romance in there, b) it's ultimately about a haunted house, and c) it has zombies. Frankenstein is horror because it is essentially about a man's attempt to achieve godhood by doing something only God has done before. Dracula is almost pure romance. It has all the elements of a classic gkthicromance and, in fact, birthed the genre imnsho.
When humans are doing the terrorizing, to me, it's either thriller or suspense.